Do we need a new api for SignedRequestAuth?

[nflaig]

we might need a new api endpoint for the validator to sign over SignedRequestAuth because right now only the beacon node has this information, the validator client needs to be able to ask the beacon node to know what to sign over

Related to ethereum/builder-specs#156

[nflaig]

we don't need this, the validator client has all information already to create a SignedRequestAuth and send it as part of produceBlockV4 to the beacon node, see #630 for details