Add finder for getting commits with a trailer set

This class fetches all commits in a range in batches, selecting only
commits with a certain trailer set. This class will be used to fetch the
commits that should be used to generate a changelog.

This builds on the changes introduced in
https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2842 and
https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2999. To take
advantage of these changes, we also bump the Gitaly Gem version and add
fallback code for getting trailers using Rugged; as Rugged is still in
use. In addition, we change the merge_request_diff_commits table and the
MergeRequestDiffCommit model to support storing the trailers as JSON.

See https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1365 for
more information.