Specify how to handle conflicting bids

This is a bit of an edge case but I think it is worth specifying that if a proposer (with the boost software acting as a proxy) receives multiple signed builder bids with the same *value* but differing payload headers, the proposer should break the tie with the hash tree root of the signed bid (e.g. choose the lessor root interpreting the root as a 256-bit number)

Review the external block-builder specification and its existing bid-selection rules first. Done means the specification documents an unambiguous tie-break for equal-value signed bids with differing payload headers. No file or test is named in the issue.

Written by the indexing model from the issue text.