Get tag signature via Tags API

What does this MR do and why?

Contributes to #384116 (closed)

Problem

We don't have an tags API to return signature the same way as we do for commits. Example of commit's signature: https://gitlab.com/api/v4/projects/72724/repository/commits/189a6c924013fc3fe40d6f1ec1dc20214183bc97/signature.

Solution

Add an endpoint that will return the tag signature.

Screenshots or screen recordings

http://127.0.0.1:3000/api/v4/projects/1/repository/tags/v1.1.1/signature

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.