lib/repo-finder: Add config-file based OstreeRepoFinder implementation

This is a basic implementation of OstreeRepoFinder which resolves ref
names to remote URIs by looking their collection IDs up in the local
configuration of remotes who have their collection-id key set.

Unit tests are included.

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

Closes: #924
Approved by: cgwalters

src/libostree/libostree-experimental.sym

@@ -47,6 +47,8 @@ global:
   ostree_collection_ref_new;
   ostree_repo_find_remotes_async;
   ostree_repo_find_remotes_finish;
+  ostree_repo_finder_config_get_type;
+  ostree_repo_finder_config_new;
   ostree_repo_finder_get_type;
   ostree_repo_finder_resolve_async;
   ostree_repo_finder_resolve_all_async;