Add way to contact users

[rolznz]

If this is a paid service we might need to get in touch with users who do not pay (e.g. if they will soon lose their lightning address).

But how do they provide this and when? and it should be optional.

[rolznz]

Update: user must provide a nostr pubkey when creating an address

[jpl-btc]

Perhaps we could make it so each hub has its own Nostr key, automatically and randomly generated. The user does not need to know that the messages are received using Nostr. We just implemented Nostr private messaging system without telling the user it is Nostr. Implementing a private messaging system in the Hub sounds like a ton of work