[Deliverable] Improve RLN UX by reducing contract interactions

[jm-clius]

Owner: core research

Feature: RLN Smart Contract

FURPS:

• U3. Application does not need to do a Web3 RPC call for every tree change to generate or validate messages.
• U4. Application can transfer tokens and register membership with a single transaction.

Checklist:

• Specs: link to specs and/or API definition
• Code: link to GitHub issues/PRs/Epic
• Dogfood: link to dogfooding session/artefact
• Docs: links to README.md or docs.waku.org (TBD)