gpg: Add OstreeGpgVerifyResult

gpg: Add OstreeGpgVerifyResult

Wrappers a referenced gpgme_verify_result_t so detailed verify results
can be examined independently of executing a verify operation.

_ostree_gpg_verifier_check_signature() now returns this object instead
of a single valid/invalid boolean, but the idea is for OstreeRepo to also
return this object for commit signature verification so it can be utilized
at the CLI layer (and possibly by other programs).