lib/repo: Add OSTREE_REPO_METADATA_REF as a well-known metadata store

As discussed in https://github.com/ostreedev/ostree/pull/946, the
summary file is becoming an unsigned cache of ref information; any
additional metadata for the repository needs to move elsewhere in order
to remain signed. Introduce OSTREE_REPO_METADATA_REF as the well-known
name of a ref where such metadata can live, as the metadata on
contentless commits.

Don’t yet update the documentation for summary-related methods to
mention this, since it’s still hidden behind the
--enable-experimental-api configure option.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #946
Approved by: cgwalters

apidoc/ostree-experimental-sections.txt

@@ -28,6 +28,7 @@ ostree_repo_find_remotes_finish
 ostree_repo_pull_from_remotes_async
 ostree_repo_pull_from_remotes_finish
 ostree_repo_resolve_keyring_for_collection
+OSTREE_REPO_METADATA_REF
 </SECTION>
 
 <SECTION>