feat (rln-relay): allow higher messaging rate using multiple valid registrations

[staheri14]

Problem

Waku-rln-relay protocol only accepts one valid rln credential for messaging in a given group. However, it is perceivable that a node wants to increase its messaging quota by using multiple valid credentials. This issue is to enable such a feature in the waku-rln-relay protocol.

Suggested solution

At a high level, the waku node should be able to accept multiple valid rln credentials and use them in a cyclic fashion so that the user benefits from his maximum messaging quota.

Additional context

Was raised in #1238 (comment)

Acceptance criteria

• A proper solution is devised and implemented in nim-waku
• the RFC gets updated as fits

[rymnc]

https://github.com/Rate-Limiting-Nullifier/rln-circuits-v2 allows this. This issue is blocked by -

• contract implementation required
• zerokit implementation required

[chair28980]

Closing per nwaku pm meeting Sep 12 '23. Can reopen if this becomes a requirement. @rymnc