core: Add detached metadata, readd metadata to commits

Previously I thought we'd have to ditch the current commit
format to avoid a{sv} due to

See https://bugzilla.gnome.org/show_bug.cgi?id=673012

But I realized that we don't really have to care about
unpacking/repacking commit objects, so let's just re-expose the
existing metadata a{sv} in commits in the API.

Also, add support for "detached" metadata that can be updated at any
time post-commit.  This is specifically designed for GPG signatures.

https://bugzilla.gnome.org/show_bug.cgi?id=707379

src/libostree/ostree-repo.h

@@ -318,12 +318,25 @@ gboolean      ostree_repo_write_commit (OstreeRepo   *self,
                                         const char   *parent,
                                         const char   *subject,
                                         const char   *body,
+                                        GVariant     *metadata,
                                         const char   *root_contents_checksum,
                                         const char   *root_metadata_checksum,
                                         char        **out_commit,
                                         GCancellable *cancellable,
                                         GError      **error);
 
+gboolean      ostree_repo_read_commit_detached_metadata (OstreeRepo      *self,
+                                                         const char      *checksum,
+                                                         GVariant       **out_metadata,
+                                                         GCancellable    *cancellable,
+                                                         GError         **error);
+
+gboolean      ostree_repo_write_commit_detached_metadata (OstreeRepo      *self,
+                                                          const char      *checksum,
+                                                          GVariant        *metadata,
+                                                          GCancellable    *cancellable,
+                                                          GError         **error);
+
 /**
  * OstreeRepoCheckoutMode:
  * @OSTREE_REPO_CHECKOUT_MODE_NONE: No special options