[Deliverable] Implement RLN smart contract for paid, multilevel memberships

[jm-clius]

Description

Implement the smart contract changes proposed as output of #205 and ensure it's reflected in the different Waku clients.
There is already an initial discussion on what features we need to support.
Once a minimal set of features is agreed for the RLN smart contract, this is the first step to move to a production contract on mainnet.

Note on scope of client support

This deliverable includes support for the new contract in the different Waku clients. Note that this deliverable only tracks minimal support to have a working RLN membership set and validation using the new contract. In other words:

• nwaku clients must point to an instance of the new contract and use that membership set for proof verification
• there must be some tool to register new RLN memberships in an instance of the new contract, even if it's a simple update to the nwaku-compose script.

Supporting more new contract features, using the on-chain tree, a web management tool, etc. are out of scope and defined in other deliverables.

[jm-clius]

cc @fryorcraken added some clarification on scope of client implementation here. LMK if you have comments.