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.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information