Add GetTagSignatures RPC
As requested in #2719 (closed), this MR implements the GetTagSignatures
RPC (analogous to the similar GetCommitSignatures
RPC).
This RPC is required to simplify and optimize the handling of tag signatures in GitLab and to fix an existing bug (the signature is integrated in the tag message, if both together are too long, the signature is truncated and thus invalid).
Closes #2719 (closed)
/cc @bufferoverflow