ci: add workflow to enforce Conventional Commit tags (#370)

Currently our release process is fairly manual: someone with write
access to this repo has to check out the latest from `main`, look at
what has changed since the last release, decide on an appropriate new
version number, create a git tag in the correct format (is it `x.y.z` or
`vx.y.z`?), and push that tag back up to GitHub to trigger the release
process.

By ensuring that our PR titles have Conventional Commit tags, we make it
possible to automatically determine the next release version number,
which means we can move to a push-button release process: click the
release button, and the workflow does the rest.

This workflow has been in place for some time on `metal-go` and was
recently adopted in `metal-java`; adopting it more broadly will make it
easier for us to safely share the responsibility for releasing our
various tools (and could even lead to automated / scheduled release
workflows instead of push-button workflows).

.github/workflows/validate_pr.yml

@@ -0,0 +1,45 @@
+name: "Validate PR"
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+
+permissions:
+  pull-requests: write
+
+jobs:
+  main:
+    name: Validate PR title
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v5
+        id: lint_pr_title
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: marocchino/sticky-pull-request-comment@v2
+        # When the previous steps fails, the workflow would stop. By adding this
+        # condition you can continue the execution with the populated error message.
+        if: always() && (steps.lint_pr_title.outputs.error_message != null)
+        with:
+          header: pr-title-lint-error
+          message: |
+            Hey there and thank you for opening this pull request! 👋🏼
+
+            We require pull request titles to follow the [Conventional Commits specification](https://www.conventionalcommits.org/en/v1.0.0/) and it looks like your proposed title needs to be adjusted.
+
+            Details:
+
+            ```
+            ${{ steps.lint_pr_title.outputs.error_message }}
+            ```
+
+      # Delete a previous comment when the issue has been resolved
+      - if: ${{ steps.lint_pr_title.outputs.error_message == null }}
+        uses: marocchino/sticky-pull-request-comment@v2
+        with:
+          header: pr-title-lint-error
+          delete: true