Allow object removal from journal

swh.journal is currently missing a way to remove objects from the journal. This would be required to properly process some takedown notices.

Implementation-wise, in Kafka this is part of the log compaction mechanism:

Compaction also allows for deletes. A message with a key and a null payload will be treated as a delete from the log. Such a record is sometimes referred to as a tombstone. This delete marker will cause any prior message with that key to be removed (as would any new message with that key), but delete markers are special in that they will themselves be cleaned out of the log after a period of time to free up space.

Should a delete_object(object_type: str, object_key: KeyType) method be added to swh.journal.writer.interface.JournalWriterInterface?

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Migrated from T4657 (view on Phabricator)