Provide an API endpoint to get the GPG signature of a commit

Problem to solve

The GPG signature of a single commit cannot be fetched from the frontend.

Further details

• We have a public API for single commits: https://docs.gitlab.com/ee/api/commits.html#get-a-single-commit
• We have an internal API for GPG signatures of a branch: curl https://gitlab.com/gitlab-org/gitlab-ce/commits/master/signatures

Proposal

Create a public API endpoint /projects/:id/repository/commits/:sha/signature or extend the existing /projects/:id/repository/commits/:sha endpoint with the GPG signature.