Extend contract to reduce number of Web3 RPC calls

[stubbsta]

Background

In the current RLN implementation, each nwaku node that needs to generate or verify a zero-knowledge proof polls the on-chain Merkle tree contract every 5 seconds to fetch the latest root. This is necessary because the root changes each time another membership is registered or erased, potentially with every new block (e.g., Linea has a 2–3s block time).
This results in a continuous stream of eth_call requests to a Web3 RPC endpoint.
This issue is defined in the roadmap and related discussions here.

Problem

The frequent polling results in a high number of RPC requests, which can negatively impact users—especially those using limited or free-tier Web3 RPC providers, or those running multiple nodes that share the same endpoint.

A bug was identified related to membership insertion time - it is currently possible for a node to receive a message with a root "from the future" which is then invalid. This may be solved with the improved WebRTC call strategy that this issue aims to achieve.

Possible Solutions

1. Add a sliding window of latest roots to the contract - added gas cost to write to the contract
2. A node is triggered for the latest root when it receives a message
3. Polling strategy - run simulations to determine what polling time is optimal and what are the factors that impact it.

[stubbsta]

The current WakuRlnV2 contract workflow for updating and retrieving the merkle tree root is:

• register function is called
  -> which calls _upsertInTree
  -> which calls the LazyIMT contract update or insert functions
  -> which updates the merkle tree

• root() function is called
  -> LazyIMT.root is called and return the latest merkle tree root

Proposed solurion (Add a sliding window of latest roots to the contract):

• Add fixed array storage with circular buffer implementation
• Add function to update the storage when the merkle tree is modified (via registration or non-lazy erase memberships function call)
• Add function to return the entire array (or root at specific index)
• Keep the current root() function for nwaku backwards compatibility (could be deprecated at some point)

[stubbsta]

Tasks:

• Updates required to WakuRlnV2 contract to add storage
• Updates required to nwaku to retrieve roots and manage the validroots list
• Run simulations to determine ideal root storage size andtest edge case scenarios