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).

[michaelpri10]

Accidentally left comments on the local repo pull request, but copied the relevant ones here. Just 2 small suggestions but otherwise LGTM.