OstreeGpgVerifier: Take the signature as a GBytes

OstreeGpgVerifier: Take the signature as a GBytes

The signature data is in memory to begin with, so there's no need to
write it to disk only to immediately read it back.

Also, because the GPGME multi-keyring workaround is somewhat expensive
to setup and teardown, concatenate all signatures into a single GBytes
so _ostree_gpg_verifier_check_signature() is only called once.  We're
currently only looking for one valid signature anyway.