buffer proxy: Add 'raw_extrinsic_metadata' to the list of buffered objects

This does not matter much for normal loaders as they only do a single raw_extrinsic_metadata_add before flushing; but the metadata loader loads metadata from a bunch of origins (as many as there are origins in the configured Kafka batch) one-by-one before flushing

Found while investigating swh/meta#5035 (closed), though that's probably not the cause of the main slowness.