Add payment proof to builder bid

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.

Start with the builder bid object and the specification for blocks N and N+1. Define how proofs of the fee_recipient balance are rooted against both headers and how validators verify the claimed payment; done means the specification fully describes this proof flow.

Written by the indexing model from the issue text.