Add finder class for fetching commits with a certain Git trailer set

What does this MR do?

This adds the finder CommitsWithTrailerFinder, which can be used to fetch commits in a range and with a certain Git trailer set.

This class will be used to obtain the commits to present in a changelog, which is worked on in gitlab-com/gl-infra/delivery#1369 (closed).

See gitlab-com/gl-infra/delivery#1365 (closed) for more information.

TODO

• Update GITALY_SERVER_VERSION to the right SHA
• Ensure a new version of the gitaly Gem is released
• Bump the gitaly Gem version in Gemfile.lock to this new version
• Yield pages of commits, instead of returning all commits. This allows for batch processing, instead of the changelog generator having to load all commits into memory

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Backstage change
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content