add eth_sendMegabundle RPC

[thegostep]

Description

eth_sendMegaBundle would allow for sending bundles which bypass the bundle scoring auction and instead are directly inserted in a block in the multi-worker

the mev-geth node is reponsible for filling the remaining space in the block with transactions from the mempool

the rpc parameters would be:

{
    "transaction_list": [],
    "timestamp": "",
    "coinbase_diff": "",
    "coinbase": "0x",
    "parent_hash": "",
}

Specification

https://docs.flashbots.net/flashbots-auction/miners/mev-geth-spec/v04

[bogatyy]

Met with @tkstanczak @jparyani @pgebheim , decided the following:

1. RLP is decidedly better than JSON (which was only used on Flashbots relay outside Geth), so definitely serializing via RLP
2. blockNumber is going to be a uint. Jason thinks it's cleaner and hex block numbers were a mistake to begin with. Ivan had a slight preference for keeping compatibility with the standard block number format (hex), but ok with uint too.
3. For RLP serialization, we will have a separate data structure. It will not have a relaySignature field, and all the fields will be mandatory (e.g. mandatory zeros for minTimestamp, maxTimestamp). Goal is to make it 100% obvious that serializations are deterministic, and not worry about nils vs zeros.

[bogatyy]

Pushed out a fix to conform to the meeting decisions above, in the PR #86

Also, as FYI, here is an example of a megabundle builder in Python https://github.com/Mining-DAO/examples/blob/main/megabundle_sender.py