Specify Bid Scoring with ePBS

With ePBS, a staked builder can make a payment to a proposer via either a trustless payment or a trusted payment or use both.

The [value](https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/beacon-chain.md?plain=1#L264) field in the [ExecutionPayloadBid](https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/beacon-chain.md?plain=1#L255) represents a trustless payment and the [execution_payment](https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/beacon-chain.md?plain=1#L265) field in the same structure represents a trusted payment.

The total value of the block could just `bid.value` + `bid.execution_payment`. But do we want to discount value from `bid.value` because the trusted payment doesn't happen immediately but gets credited to the validator at the end of every epoch.

Start with specs/gloas/beacon-chain.md, especially the ExecutionPayloadBid definition and its value and execution_payment fields. Determine whether bid scoring should sum both payments or discount the trusted payment because it is credited at epoch end; done means the scoring rule and its timing are specified clearly.

Written by the indexing model from the issue text.