lib: Add an API to GPG verify a commit given a remote

Conceptually we've been moving towards having our GPG verification
paths be per-remote.  The code internally supports this, but we
didn't expose an API to use it conveniently.

This came up when trying to add a new `gpgkeypath` option, since
right now rpm-ostree manually finds keyrings for the remote, and
hence it wasn't looking at the keypath, and said "Unknown key"
in status.

Adding an API fixes this nicely.

Closes: #576
Approved by: giuseppe

apidoc/ostree-sections.txt

@@ -379,6 +379,7 @@ ostree_repo_add_gpg_signature_summary
 ostree_repo_gpg_verify_data
 ostree_repo_verify_commit
 ostree_repo_verify_commit_ext
+ostree_repo_verify_commit_for_remote
 ostree_repo_verify_summary
 ostree_repo_regenerate_summary
 <SUBSECTION Standard>