[native_assets_cli] Add example for downloading assets

[dcharkes]

Add an example for how to download assets in a hook which are built on GitHub CI.

Design approach:

• tool/build.dart uses package:native_toolchain_c.
• GitHub action that publishes artifacts on a tag (not the package release tag).
• hook/build.dart downloads the artifacts.
• tool/generate_asset_hashes.dart generates hashes that are used in hook/build.dart to check download integrity.

GitHub actions mostly cargo culted from:

• https://github.com/dart-lang/i18n/blob/83e8d08568e26888dc0252c89f5d1faf92052e44/.github/workflows/intl4x_artifacts.yml

Closes: #157

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 88.026% (+0.3%) from 87.717%
when pulling 16d4e33 on download-example
into a646680 on main.

[mosuem]

The name is a bit misleading, maybe building_assets?

[dcharkes]

Hm, yeah, it's the name of the example package, because we're in a mono repo here. I agree with you that if it's not a mono repo it should probably be named building_assets. Should we name it package_download_asset.yaml and name: package_download_asset?

[mosuem]

Sure, but it's just a nit. So do as you want :)

[mosuem]

Nit: I would print the hashes