Move endpoint to get unsigned bid to builder namespace

[nflaig]

The endpoint GET /eth/v1/validator/execution_payload_bid/{slot}/{builder_index} is already part of the spec but instead of being part of /validator it would be better to move it to /builder as this endpoint will only be used by staked builders, not validators.

[nflaig]

flagged by Potuz, we might also wanna consider allowing to pass execution_payment if builders wanna use this