Allow finding commits by message at FindCommitsRequest (deprecate CommitsByMessageRequest)

Seems like the main difference between CommitsByMessageRequest and FindCommitsRequest is that CommitsByMessageRequest is mostly locked to searching commits, well, by messages.

Would that make sense to add a Query param to FindCommitsRequest in order to deduplicate these and allow searching for a specific author and a specific commit message (gitlab!16683 (closed))?

After that I can't see a reason to keep using CommitsByMessageRequest at https://gitlab.com/gitlab-org/gitlab/blob/8f8e2333b4921f820e7ca7abf5e25280a7c4730f/lib/gitlab/gitaly_client/commit_service.rb#L245-245 so I think it could be deprecated.