[FR] Add method PGPPayload() for tag and commit

[sapk]

Before doing a PR I would like to know your if it would be accepted and what it should look like to be accepted.

Currently only the full Encode method of a commit/tag is exposed publicly. In the goal of being able to verify the payload directly from a keyring and not a armored key string like the Verify method of tag and commit, I would suggest to add another method like PGPPayload() string that export the result of t.encode(o, false). This method could also be re-use in the Verify of commit and tag.

I mainly ask because it doesn't follow the contributing rule: You should be able to run the same query using git. We don't accept features that are not implemented in the official git implementation. as I don't think we can export without the sig with the git cli. But this method would be very useful and limit the duplication of code in and outside go-git to do the same.

[filipnavara]

Notably it's something we run into in Gitea. The GitHub API exposes the PGP payload to the users (https://developer.github.com/v3/git/commits/) and libgit2 has an API to get it (https://libgit2.org/libgit2/#HEAD/group/commit/git_commit_extract_signature).

[sapk]

Closed by #1127