Optimistic builder should only build if it is going to be the next proposer

[shotes]

Currently every single validator node optimistically sends a build over the engine API for every single slot. This workload can be greatly reduced by only optimistically building if the validator thinks it is going to be the next proposer.

There needs to be a feasibility analysis done to determine the consistency of determining when a validator is going to be the next proposer. Additionally, this needs to ensure proper calculation under faulty conditions. For example, we must be able to determine the next proposer accurately (and trigger timely builds) when there are multiple failed rounds for a slot.