add ability to generate transactions for testing

[ralexstokes]

want a component that can be configured to do something kind of like this: https://github.com/MariusVanDerWijden/tx-fuzz

although it can start even simpler:

every slot, just inject a valid transaction that transfers ETH around with a high priority fee

the EL in this case will need to be configured to not gossip transactions from the local mempool, which in practice means keeping it unpeered from other ELs on the current network -- this makes syncing more fragile but is an acceptable compromise for now

[PatStiles]

@ralexstokes this could be a little left field but expanding this to support property testing and state machine testing for the relay/builder could be a interesting.

[ralexstokes]

yeah that would be neat!

another thing to think about is something like mev-flood... I don't know if we want to "rewrite it in rust" but it would be useful for testing