lib/repo-finder: Add OstreeRepoFinderOverride

This is another OstreeRepoFinder implementation; it returns results from
a given set of URIs. It’s designed to be used for implementing user
overrides to other repo-finders, or for implementing unit tests.

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

Closes: #1281
Approved by: mwleeds

src/libostree/libostree-experimental.sym

@@ -82,3 +82,10 @@ LIBOSTREE_2017.12_EXPERIMENTAL {
 global:
   ostree_repo_resolve_collection_ref;
 } LIBOSTREE_2017.8_EXPERIMENTAL;
+
+LIBOSTREE_2017.13_EXPERIMENTAL {
+global:
+  ostree_repo_finder_override_add_uri;
+  ostree_repo_finder_override_get_type;
+  ostree_repo_finder_override_new;
+} LIBOSTREE_2017.12_EXPERIMENTAL;