feat: add signature type to GitCommit

This MR adds a new enum SignatureType to shared.proto which is used by the Commit and Tag message. The idea behind is to get the type of the signature as early as possible and to reduce number of calls from gitlab-rails. This would e.g. allow to avoid commit_service#get_commit_signatures calls for commit without any signature as they got signature_type: :NONE already by commit_service#find_commit at an early stage.

related to gitlab!17773 (comment 244895120)