Add a way to reject ingesting documents from certain users during sync

[sgwilym]

What's the problem you want solved?

Right now peers always ingest all documents they receive from another peer.

Sometimes you don't want all documents from another peer, as maybe they are sending documents containing abuse or harassment. It should be possible to configure a replica / the sync process in such a way that it rejects documents authored by a certain keypair.

This isn't a permanent solution, as an abuser can always create a new keypair with a new address.

Is there a solution you'd like to recommend?

Add a kind of syncing policy configuration to syncers which can specify a blocklist of author addresses.