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.

src/libostree/ostree-gpg-verifier.h

@@ -51,6 +51,12 @@ OstreeGpgVerifyResult *_ostree_gpg_verifier_check_signature (OstreeGpgVerifier *
                                                              GCancellable      *cancellable,
                                                              GError           **error);
 
+gboolean      _ostree_gpg_verifier_list_keys (OstreeGpgVerifier   *self,
+                                              const char * const  *key_ids,
+                                              GPtrArray          **out_keys,
+                                              GCancellable        *cancellable,
+                                              GError             **error);
+
 gboolean      _ostree_gpg_verifier_add_keyring_dir (OstreeGpgVerifier   *self,
                                                     GFile               *path,
                                                     GCancellable        *cancellable,