librarian: [release] Implement release tag-and-release Command #1009
Description
Build the CLI for the release tag-and-release subcommand, including the logic to find merged PRs, create Git tags, create GitHub Releases, and update PR labels.
A release commit message structure depends on the tagging strategy configured for the repository.
Tag Per Repo
In this strategy, a single release pull request results in one release tag for the entire repository, which is suitable for languages like Java and C++. The Librarian will determine the version and other variables to fill in the template.
chore(main): release {version}
Librarian Version: {librarian_version}
Language Image: {language_image_name_and_digest}
<details><summary>{version}</summary>
[<version>](https://github.com/googleapis/{repo}/compare/{previous_tag}...{new_tag}) ({date})
### Features
* {commit_summary} ([{commit_hash}](https://github.com/googleapis/{repo}/commit/{commit_hash}))
### Bug Fixes
* {commit_summary} ([{commit_hash}](https://github.com/googleapis/{repo}/commit/{commit_hash}))
### Documentation
* {commit_summary} ([{commit_hash}](https://github.com/googleapis/{repo}/commit/{commit_hash}))
</details>
Tag Per Library
In this strategy, a single release pull request can generate multiple, distinct tags, one for each library that has changed. This approach is used by languages like Python and Go. A section for each library release will be wrapped into a <details> element, where the <summary> child element will contain the release tag.
chore: release main
Librarian Version: {librarian_version}
Language Image: {language_image_name_and_digest}
<details><summary>{library_name}: {version}</summary>
[{version}](https://github.com/googleapis/{repo}/compare/{library_name}-v{previous_version}...{library_name}-v{new_version}) ({date})
### Features
* {commit_summary} ([{commit_hash}](https://github.com/googleapis/{repo}/commit/{commit_hash}))
</details>
<details><summary>{another_library_name}: {version}</summary>
[{version}](https://github.com/googleapis/{repo}/compare/{another_library_name}-v<previous_version>...{another_library_name}-v{new_version}) ({date})
### Bug Fixes
* {commit_summary} ([{commit_hash}](https://github.com/googleapis/{repo}/commit/{commit_hash}))
</details>
Librarian will parse the commit message and create a separate release tag for each <details> section.
In addition to creating a release tag per library, Librarian will create a release-please-{PR number} tag that is used for triggering a Louhi release flow that releases multiple libraries at once.