Add payment proof to builder bid

[lightclient]

Right now we're trusting the bid from the builder will pay the value it claims. This can and should be provable with a state proof.

Suppose we're building block N+1 -- the builder bid object can be extended to have a proof to the balance of the fee_recipient at block N and N+1. The proofs would be rooted against the headers for N and N+1. This would allow the validator to verify that their account does receive the value as expected.

[metachris]

See also flashbots/mev-boost#99 (comment) for more discussion and context about different models for payment proofs.

Two discussed methods:

1. inclusion proof on transaction trie
2. account proof on post execution state trie