Disable mailmap for `GetCommitSignatures`

Context: https://gitlab.com/gitlab-org/gitlab/-/issues/425042#note_1997022896

The GetCommitSignatures RPC returns the signatures and signee for a list of commits. But for commits signed by GitLab, if mailmap is enabled, the verification in Gitaly fails. This is because git-cat-file will provide the mapped author and the verify stage will compare against the original signature which holds the unmapped author.

In this this, it doesn't make sense to use mailmap. Let's disable it for this particular RPC. To also help clients understand which author the signature pertains too, let's add a new field to the RPC providing the author details.

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