lib/repo: Add collection ID support to OstreeRepo

Add {get,set}_collection_id() methods to OstreeRepo and some documentation
about the concept of a collection ID which globally identifies an
upstream repository. See the documentation for more details.

This will be used in future commits. For now, the new API is marked as
experimental (--enable-experimental-api).

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

Closes: #924
Approved by: cgwalters

src/libostree/ostree-repo-private.h

@@ -20,6 +20,7 @@
 
 #pragma once
 
+#include "ostree-ref.h"
 #include "ostree-repo.h"
 #include "ostree-remote-private.h"
 #include "libglnx.h"
@@ -129,6 +130,7 @@ struct OstreeRepo {
   gboolean enable_uncompressed_cache;
   gboolean generate_sizes;
   guint64 tmp_expiry_seconds;
+  gchar *collection_id;
 
   OstreeRepo *parent_repo;
 };
@@ -349,4 +351,13 @@ _ostree_repo_get_remote_inherited (OstreeRepo  *self,
                                    const char  *name,
                                    GError     **error);
 
+#ifndef OSTREE_ENABLE_EXPERIMENTAL_API
+
+const gchar * ostree_repo_get_collection_id (OstreeRepo   *self);
+gboolean      ostree_repo_set_collection_id (OstreeRepo   *self,
+                                             const gchar  *collection_id,
+                                             GError      **error);
+
+#endif  /* !OSTREE_ENABLE_EXPERIMENTAL_API */
+
 G_END_DECLS