Add an unmailmapped committer email to Gitaly GetCommitSignatures RPC

What

Add an unmailmapped committer email to the getCommitSignatures RPC.

Why

Previously the unmailmapped author email was added to the GetCommitSignatures response: gitaly!7166 (merged). Following a conversation https://gitlab.com/gitlab-org/gitlab/-/issues/425042#note_2506140631 it was recognised that we should be checking for mailmap committer emails not author emails. We should update the Gitlay RPC response to contain unmailmapped committers not authors to allow Gitlab rails to do that.