Add invite code functionality

[sanah9]

• Store invite codes in state with expiration support
• Add invite code verification for kind 9021 join requests
• Automatic cleanup of expired invite codes

[fiatjaf]

@staab please review

[staab]

I would personally have the relay generate the code and expiration policy rather than the requester. This is how https://github.com/nostr-protocol/nips/pull/1079/files works. But it's completely unspecified, so this approach is also fine. The code and expiration stuff should probably be optional, and possible for the relay to reject if they fall outside of certain boundaries. But the PR looks fine to me.

[fiatjaf]

Unconvincing review, I'll have to review it myself.

[purrgrammer]

found this while trying to figure out how invite codes work for nip-29 groups. lgtm, we should probably update the NIP-29 spec since as @staab mentions the invite codes are underspecified.