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/ostree.h

@@ -38,6 +38,7 @@
 #ifdef OSTREE_ENABLE_EXPERIMENTAL_API
 #include <ostree-ref.h>
 #include <ostree-repo-finder.h>
+#include <ostree-repo-finder-config.h>
 #endif /* OSTREE_ENABLE_EXPERIMENTAL_API */
 
 #include <ostree-autocleanups.h>