Add ability to direct new transactions to specific endpoint(s)

[boxhock]

A use case for L2s is where you have multiple L2 replica nodes running, which don't support broadcasting txs. You then want to use this Proxy for the replica nodes, while any eth_sendRawTransaction requests will go to a designated tx broadcasting endpoint (e.g. official endpoint).

[yorickdowne]

This is no longer needed for Optimism since l2geth 0.5.8, but could still be useful for other L2s - TBD.