lib/repo: Add ostree_repo_remote_get_gpg_keys()

lib/repo: Add ostree_repo_remote_get_gpg_keys()

This function enumerates the trusted GPG keys for a remote and returns
an array of `GVariant`s describing them. This is useful to see which
keys are collected by ostree for a particular remote. The same
information can be gathered with `gpg`. However, since ostree allows
multiple keyring locations, that's only really useful if you have
knowledge of how ostree collects GPG keyrings.

The format of the variants is documented in
`OSTREE_GPG_KEY_GVARIANT_FORMAT`. This format is primarily a copy of
selected fields within `gpgme_key_t` and its subtypes. The fields are
placed within vardicts rather than using a more efficient tuple of
concrete types. This will allow flexibility if more components of
`gpgme_key_t` are desired in the future.