Get Signature Verification When Viewing Git Data Tags

[shiftkey]

API documentation: https://developer.github.com/v3/git/tags/#tag-signature-verification

An enhancement as part of the Commit Signing API is to surface the signature details in the tag.

It looks like these are the new properties:

    "verification": {
      "verified": true,
      "reason": "valid",
      "signature": "-----BEGIN PGP MESSAGE-----\n...\n-----END PGP MESSAGE-----",
      "payload": "tree 6dcb09b5b57875f334f61aebed695e2e4193db5e\n..."
    }

These fields should be available on the GitTag response object (perhaps in a class called Verification or something), and the TagsClient.Get action should be updated to use the new Accepts header:

application/vnd.github.cryptographer-preview+sha

I'd also like to see some new integration tests added to verify this behaviour is working as expected.

[mderriey]

Hey,

I'd like to give this one a shot.
From the looks of it, what has been done in #1398 can be reused.

Cheers

[shiftkey]

@mderriey sounds good!