repo: Add functions for verifying any data in repository

This can be useful for validating the 3rd party data that is put in
the extensions directory and is signed with the same keys as commits
or the summary file.

Closes: #310
Approved by: cgwalters

src/libostree/libostree.sym

@@ -342,5 +342,6 @@ LIBOSTREE_2016.6 {
 global:
         ostree_gpg_verify_result_require_valid_signature;
         ostree_raw_file_to_archive_z2_stream;
+        ostree_repo_gpg_verify_data;
         ostree_repo_remote_fetch_summary_with_options;
 } LIBOSTREE_2016.5;