Port GetCommitSignatures to Go
The GetCommitSignatures is a RPC that is located in Gitaly-Ruby right now, but gets quite some traffic. It would be nice to port it to Go, and with that get a suspected performance improvement.
The go implementation should be added behind a feature flag. (Example MR: !1106 (merged)) The feature flag is set by GitLab, so very minor work on GitLab is required too.