Add flashbots_ rpc namespace support

[mattsse]

ref https://docs.flashbots.net/flashbots-auction/searchers/advanced/rpc-endpoint#flashbots_getuserstatsv2

TODO

• add types + rpc trait, see existing eth/mev.rs

[optimiz-r]

I'm down to dive into this to get to know the repo better and gear up for #29.
If anybody's already on it, give me a shout. Otherwise, I'm on it from tomorrow.

[mattsse]

nice,

fyi the userstatsv2 type is already available