feat(attestation): add git repository annotations

[migmartri]

Adds additional annotations to the git.head subject in the attestation. These annotations include the commit message, author, timestamp and remotes.

      "name": "git.head",
      "digest": {
        "sha1": "78ac366c9e8a300d51808d581422ca61f7b5b721"
      },
      "annotations": {
        "author.email": "john@cyberdyne.com",
        "author.name": "John Connor",
        "date": "2023-10-26T17:09:20Z",
        "message": "chatGPT wrapper",
        "remotes": [
           {
             "name": "upstream",
             "url": "git@cyberdyne.internal:skynet.git"
           }
         ]
      }
    }

Closes #383

[danlishka]

love it!