allow users to send transactions via flashbots or other private transaction service

[moodysalem]

Is your feature request related to a problem? Please describe.
user transactions can be frontrun, resulting in them receiving a worse price than quoted up to their slippage tolerance

Describe the solution you'd like
users should be able to submit transactions to private mempools where the fee is negotiated directly with the private mempool in order to avoid their trades being sandwiched when they go into the public mempool

Additional context
this protection is not perfect. some transactions can still be sandwiched due to reorgs after the transaction is included. this will also likely need a router change. https://github.com/Uniswap/uniswap-v3-periphery/issues/165

see https://hackmd.io/@flashbots/use-cases