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

tests/.gitignore

@@ -14,4 +14,5 @@ test-mutable-tree
 test-ot-opt-utils
 test-ot-tool-util
 test-ot-unix-utils
+test-repo-finder-config
 test-rollsum-cli