common/archive: Ensure backward compatibility with swh-indexer 1.x

idx_storage.origin_intrinsic_metadata_get does not return the same
dict fields according to the swh-indexer version used.

As swh-indexer 2.0.0 is not yet deployed in production, we must ensure
backward compatibility with swh-indexer 1.x or the Web API endpoint
/origin/metadata-search/ will break.