This is, in large part, a refactoring: we rename `CommitVerification` to
`ObjectVerification`, and adjust `ParseObjectWithSignature` (previously
`ParseCommitWithSignature`) to work on an object, rather than a commit.
This in turn, lets us implement `ParseTagWithSignature` on top of it, so
commit & tag signature verification will share most of the code.
Work sponsored by @glts.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
