feat: enable hermetic library generation

[diegomarquezp]

This is part of the effort to enable multi-version support in hand-written libraries. More context and details in this doc.

This PR adds two workflows and their scripts:

• update_generation_config which updates generation_config.yaml (added in this PR) to contain the latest googleapis_committish (source of truth of library generation) as well as other dependencies such as gapic_generator_version. This job runs nightly and creates a PR that modifies the config yaml.
• hermetic_library_generation which reads generation_config.yaml and pushes a new commit to any PR whose latest commit affects generation_config.yaml (e.g. the nightly PR created by update_generation_config). It essentially translates changes in the config yaml to changes in the source code (of the generated part). This workflow will be a required check.

As an example of how they work in practice:

1. Once per night, update_generation_config creates a PR that updates generation_config.yaml with the latest dependencies and googleapis committish
2. hermetic_library_generation is triggered by this PR and generates the corresponding code changes. A new commit with the code changes is added to the PR. This step may occur for any PR affecting generation_config.yaml.

Follow ups

• Add CLOUD_JAVA_BOT_TOKEN to repository for the hermetic generation workflow to work. We confirmed it works in our fork (job link).

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[diegomarquezp]

/gcbrun