Add support for handling invite codes

[tionis]

It would be useful for the state interface also to track invite codes.
Currently, it seems like a relay developer has to do that manually using internal querying.
This would be really useful to e.g. show an invite code on startup, so some initial users can join auto-created groups.

[fiatjaf]

I don't know what are invite codes. Is there a protocol for them?

[tionis]

I mean the codes that can be specified in kind:9021 events as specified by nip-29.
Per the document, they can be created using a kind:9009 create-invite event.
It would be nice if the state object would track such events, so that developers have easier access to a list of currently valid invite codes with less housekeeping.

[fiatjaf]

I see, yes, you're right.

Thank you, I'll work on that eventually.