[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.