lib/ref: Add OstreeCollectionRef type for globally unique refs

This is a type representing the tuple (collection ID, ref name), which is
guaranteed to be globally unique. It will be used in upcoming commits.

It introduces the concept of a ‘collection’ which is a unique, curated
set of refs which lie in the same trust domain (i.e. all signed by the
same key and validated by the same developer). Flathub might be a
collection, for example; or the set of OS refs coming from a particular
OS vendor.

It includes a function for validating collection IDs.

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

Closes: #924
Approved by: cgwalters

src/libostree/ostree-core.h

@@ -242,6 +242,11 @@ int ostree_cmp_checksum_bytes (const guchar *a, const guchar *b);
 _OSTREE_PUBLIC
 gboolean ostree_validate_rev (const char *rev, GError **error);
 
+#ifdef OSTREE_ENABLE_EXPERIMENTAL_API
+_OSTREE_PUBLIC
+gboolean ostree_validate_collection_id (const char *collection_id, GError **error);
+#endif /* OSTREE_ENABLE_EXPERIMENTAL_API */
+
 _OSTREE_PUBLIC
 gboolean ostree_validate_remote_name (const char *remote_name, GError **error);