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

Merged vlorentz requested to merge buffer into master

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, though that's probably not the cause of the main slowness.

Edited by vlorentz

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