Send write RPC requests to a safe upstream

Problem to be solved

If the stack use takes off, we'll end up with write transactions, potentially carrying mev, being upstreamed to erpc's configured nodes. In light mode, thats a remote full node, in full mode, that's a local one. In both cases, this might have a public mempool that leads to mev-extraction.

We should use eRPCs configuration, to only send eth_call and eth_send and eth_sendRawTransaction eth_sendPrivateTransaction eth_sendBundle etc to safe, mev-rebating upstreams.

Proposed solution

We could use buildernet upstreams, mev-protect upstreams, or other options. One that i think might be simple to get started with is blinklabs.xyz. Ask for an API key from the team, and configure eRPC to disallow the write RPC calls on its unsafe upstreams, and push them all to the safe upstream.

https://docs.blinklabs.xyz/blink/get-started/integration/general
https://docs.erpc.cloud/config/projects/upstreams

Start with the eRPC upstream configuration documentation and Blink's integration guide linked in the issue. Identify where this repository configures eRPC for light and full modes, then define the safe upstream and verify that write RPC methods are routed only there; the issue names no files or tests for validation.