Add spec for proposer preferences

[nflaig]

This is currently low prio since ethereum/consensus-specs#4777 won't be part of devnet-0.

We probably want an endpoint to retrieve proposer preferences from op pool and a new event proposer_preferences which can be subscribed to by builders.

We also need a new api for validator clients to submit SignedProposerPreferences to the beacon node to be broadcast.

[nflaig]

it make sense to revisit #435 for gloas since we will need a new validation registration endpoint anyways

[nflaig]

• new spec change relevant to this Allow same epoch proposer preferences consensus-specs#5035

[nflaig]

• probably also worth to add a note related to Add pre-fork subscription note for proposer_preferences topic consensus-specs#4947 on the beacon-api