feat(rln): fetch/cache merkle proofs/roots in background

[adklempner]

Description

Generating an RLN proof requires a merkle proof of the user's commitment in the on-chain tree.

As new memberships are registered and the root is updated, the proof can become invalid per the acceptable_root_window_size

If the necessary RPC requests (to either get the proof, or check if proof is still valid) are performed on-demand per send, then this increases the time it takes to send a message.

User Story

As a user, I don't want the technical requirements imposed by RLN to increase the time it takes to send a message.

Proposed Solution / Feature Design

• After a Waku node is started and an RLN identity is "loaded", perform the RPC read request to get the latest merkle proof and root for the identity and store it.
• In the background, periodically check if a new proof/root is required per network parameters and get it if necessary. (More likely leverage eth_subscribe; event-based instead of polling-based approach)
• RLN encoder should get the merkle proof/root from a cache updated by this service.

Optional: Diagram or Draft of Design

Notes