Add builder_boost_factor to blockV3 validator API

[zilm13]

Description

See appropriate blockV3 API section: https://github.com/ethereum/beacon-APIs/blob/master/apis/validator/block.v3.yaml#L46-L78
The feature is similar to our validators-builder-bid-challenge-percentage but since the change validator will be able to set it via blockV3 call for any block production request.
We should also decide whether blockV3 overrides BN settings (if any).

[tbenr]

related to #7779

[tbenr]

@zilm13 I think values coming from VC should override BN value, which I consider it as a default.
VC represents the staker, which have the ultimate right to decide how the flow should behave.