fix: common rpc endpoint.

[darshankabariya]

I noticed that when we create a membership using rln.waku.org, it doesn’t ask for an RPC endpoint. This likely means it’s using a shared or default endpoint to handle membership requests.

That’s fine if only a few RPC calls are involved, but with wakunode, there are many RPC calls now—for roots and merkleProof. It would be better to use the user’s own endpoint instead.