Usage of the BLS signature scheme should mandate a "proof-of-possession" check

there is a well-known attack on the BLS signature scheme called a "rogue public key" attack

you can read more about it here: https://hackmd.io/@benjaminion/bls12-381#Rogue-key-attacks

the mitigation is straightforward: publish a "proof of possession" along w/ the public key.

given that this spec current requires builders to sign over their messages, we should also specify that builders publish a "proof-of-possession" alongside their public key and any other configuration info required to connect.

concretely, the "proof-of-possession" can just sign over the message that is the encoding of the builder's BLS public key according to the SSZ spec defined in this repo: https://github.com/ethereum/consensus-specs

Start by reading the builder-specs specification and the referenced consensus-specs SSZ encoding definition. Confirm how builders publish their BLS public keys and configuration, then specify the proof-of-possession requirement and its signed message alongside that information. Done means the specification clearly mandates the check and defines the required encoding.

Written by the indexing model from the issue text.