Switch to file-per-change changelog (#3774)

## Motivation and Context
Implements [RFC: File-per-change
changelog](https://smithy-lang.github.io/smithy-rs/design/rfcs/rfc0042_file_per_change_changelog.html#rfc-file-per-change-changelog)

## Description
When this PR is merged to `main`, it will impact developer workflow for
creating a changelog entry that goes with a PR.
Prior to this, one edited `CHANGELOG.next.toml` to enter a changelog
entry, but it often caused merge conflicts among us. With this PR, one
creates a `.md` file (any filename stem will do) under the
`smithy-rs/.changelog` directory. This Markdown file has the YAML front
matter, and it can be created manually or via the `ChangeLogger` CLI
(see [this
commit](83cda76)
for more details).

Here are the highlights of the code changes:
- [The release
script](https://github.com/smithy-lang/smithy-rs/pull/3774/files#diff-162dc2b82eb3d72b105291d2fc5fc8d10c4fbc1e6f35862bc08ac0d6a7c53f76)
has been pointed to `smithy-rs/.changelog`. This is pretty much what it
takes to let the `smithy-rs` release infra switch to the new changelog
mode.
-
[sdk-lints](efcc772)
now disallows `CHANGELOG.next.toml`. It will complain if the file is
present.
- The `--source-to-truncate` option for the `changelogger`'s `render` is
now optional because our internal release infra code no longer needs to
specify that option (i.e. there is nothing to truncate there).
- Tests in `smithy-rs-tool-common` and `changelogger` have been updated
so TOML changelog entries used in tests have been re-written to Markdown
changelog entries. We want to avoid giving the false impression that
TOML changelog entries are still well-supported.

## Testing
- Existing tests & lints in CI
- Ran `smithy-rs` [dry-run
release](https://github.com/smithy-lang/smithy-rs/actions/runs/10100442953)
and confirmed the correctness of `CHANGELOG.md` and
`SDK_CHANGELOG.next.json` in the release artifacts
- Ran the internal release and confirmed `CHANGELOG.md` was rendered
correctly in `aws-sdk-rust`

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

.changelog/.example

@@ -0,0 +1,10 @@
+# Example changelog entry, Markdown with YAML front matter
+# ---
+# applies_to: ["client", "server", "aws-sdk-rust"] # "aws-sdk-rust" here duplicates this entry into release notes in `aws-sdk-rust`
+# authors: ["rcoh"]
+# references: ["smithy-rs#920"]
+# breaking: false
+# new_feature: false
+# bug_fix: false
+# ---
+# Fix typos in module documentation for generated crates

.changelog/1722464864.md

@@ -0,0 +1,14 @@
+---
+applies_to:
+- aws-sdk-rust
+- client
+authors:
+- landonxjames
+references:
+- smithy-rs#3765
+- smithy-rs#3757
+breaking: false
+new_feature: false
+bug_fix: true
+---
+Fix incorrect redaction of `@sensitive` types in maps and lists.

.changelog/1722464947.md

@@ -0,0 +1,12 @@
+---
+applies_to:
+- client
+authors:
+- ysaito1001
+references:
+- smithy-rs#3767
+breaking: false
+new_feature: false
+bug_fix: true
+---
+Fix client error correction to properly parse structure members that target a `Union` containing that structure recursively.

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,8 +12,8 @@
 
 ## Checklist
 <!--- If a checkbox below is not applicable, then please DELETE it rather than leaving it unchecked -->
-- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the smithy-rs codegen or runtime crates
-- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS SDK, generated SDK code, or SDK runtime crates
+- [ ] For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the `.changelog` directory, specifying "client," "server," or both in the `applies_to` key.
+- [ ] For changes to the AWS SDK, generated SDK code, or SDK runtime crates, I have created a changelog entry Markdown file in the `.changelog` directory, specifying "aws-sdk-rust" in the `applies_to` key.
 
 ----
 

CONTRIBUTING.md

@@ -34,7 +34,9 @@ To send us a pull request, please:
 2. Modify the source; please focus on the specific change you are contributing. If you also reformat all the code, it will be hard for us to focus on your change.
 3. Ensure local tests pass.
 4. Commit to your fork using clear commit messages.
-5. Send us a pull request, answering any default questions in the pull request interface.
+5. Send us a pull request, answering any default questions in the pull request interface. To create a changelog entry Markdown file in the `.changelog` directory, you can do either of the following:
+   - Use the `new` subcommand of [changelogger](https://github.com/smithy-lang/smithy-rs/tree/main/tools/ci-build/changelogger) CLI (**preferred**)
+   - Create one manually. Name the file `XXX.md`, where `XXX` can be any name you choose, as long as it is unique in the `.changelog` directory. Ensure the contents follow Markdown syntax with the YAML front matter. For reference, see [.example](https://github.com/smithy-lang/smithy-rs/blob/3e250cf9f61ee17ccd66e16314d4e47f3dd95e25/.changelog/.example) or other Markdown files in the `.changelog` directory.
 6. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
 7. Ask maintainers to manually trigger [canary](https://github.com/smithy-lang/smithy-rs/actions/workflows/manual-canary.yml) and [PR bot](https://github.com/smithy-lang/smithy-rs/actions/workflows/manual-pull-request-bot.yml) workflows using your pull request number. Those workflows cannot run in your fork and are always skipped as such.
 

design/src/rfcs/rfc0042_file_per_change_changelog.md

@@ -1,7 +1,7 @@
 RFC: File-per-change changelog
 ==============================
 
-> Status: RFC
+> Status: Implemented
 >
 > Applies to: client and server
 
@@ -119,15 +119,15 @@ entry on behalf of a contributor).
 Changes checklist
 -----------------
 
-- [ ] Refactor changelogger and smithy-rs-tool-common to separate the changelog
+- [x] Refactor changelogger and smithy-rs-tool-common to separate the changelog
       serialization format from the internal representation used for rendering and splitting.
-- [ ] Implement deserialization for the new Markdown entry format
-- [ ] Incorporate new format into the `changelogger render` subcommand
-- [ ] Incorporate new format into the `changelogger split` subcommand
-- [ ] Port existing `CHANGELOG.next.toml` to individual entries
-- [ ] Update `sdk-lints` to fail if `CHANGELOG.next.toml` exists at all to avoid losing
+- [x] Implement deserialization for the new Markdown entry format
+- [x] Incorporate new format into the `changelogger render` subcommand
+- [x] Incorporate new format into the `changelogger split` subcommand
+- [x] Port existing `CHANGELOG.next.toml` to individual entries
+- [x] Update `sdk-lints` to fail if `CHANGELOG.next.toml` exists at all to avoid losing
       changelog entries during merges.
-- [ ] Dry-run test against the smithy-rs release process.
-- [ ] Dry-run test against the SDK release process.
+- [x] Dry-run test against the smithy-rs release process.
+- [x] Dry-run test against the SDK release process.
 
 [Changes Checklist]: #changes-checklist