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

tests/test-commit-sign.sh

@@ -80,6 +80,8 @@ mkdir repo
 ${CMD_PREFIX} ostree --repo=repo init
 ${CMD_PREFIX} ostree --repo=repo remote add origin $(cat httpd-address)/ostree/gnomerepo
 ${CMD_PREFIX} ostree --repo=repo pull origin main
+${CMD_PREFIX} ostree --repo=repo show --gpg-verify-remote=origin main | grep -o 'Found [[:digit:]] signature' > show-verify-remote
+assert_file_has_content show-verify-remote 'Found 1 signature'
 rm repo -rf
 
 # A test with corrupted detached signature