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

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