Enable the journal-writer for the swh-idx-storage in production

the config is like for swh-storage, so something like this:

      journal_writer:
        cls: kafka
        args:
          brokers: "%{alias('swh::deploy::journal::brokers')}"
          prefix: "%{alias('swh::deploy::journal::prefix')}"
          client_id: "swh.indexer.storage.journal_writer.%{::swh_hostname.short}"
          producer_config:
            message.max.bytes: 1000000000

It's unclear what the prefix should be. swh.storage uses swh.journal.objects, we can either use that one too, or a new one, eg. swh.journal.indexed

---

Migrated from T2780 (view on Phabricator)