Bounce nip-04 DMs

Currently when you try to send a kind 4 DM across the bridge, it just gets ignored. It would be nice to auto reply with "Direct messages are not supported by the bridge".

Initially I figured it was just a matter of listening for kind 4 events and then sending the above auto reply.

But it turns out my relay nostr-rs-relay actually implements NIP-04 well by not revealing the existence of kind 04 messages to unauthorised users.

One solution could be to do a NIP-42 AUTH for every generated mirror Fediverse account. Scaling issues aside, NIP-42 seems to implicitly only have a single authenticated user. That's certainly how nostr-rs-relay is implemented.

So in practice we should probably just recommend that clients disable DM functionality to proxied users.