beacon/engine: add shouldOverrideBuilder to payload envelope

[lightclient]

The shouldOverrideBuilder field was added to response of NewPayload with the intention that EL clients could give the CL client some indication of the health of the execution network in terms of censorship.

This PR is adds support for the field, but sets the value statically to zero so the CL will never override the builder. Marius has a more complete solution in #26829.

I've been using this proposed PR for testing the devents. We should make a decision on which approach to move forward with.

[rjl493456442]

Do we reach the agreement that either to return the indicator in GetPayload, or a separate engine API?

Ref: ethereum/execution-apis#425

[lightclient]

I think it already agreed upon to return the value in the GetPayload response. It's more that we need to decide if we will return false always or do something smarter like Marius' PR.

[rjl493456442]

I think the indicator returned in Marius's PR is not very useful. I don't think the censored transactions will occur in the reorg in the first place, but stuck in the txpool forever instead.

But yeah, It's not relevant with this PR though. If we agree to return this extra field in GetPayload, then it's fine for me.

[lightclient]

Yep it is in the dencun spec to return this value. I think this should be merged in the meantime while we seek out a solution which actually does let the CL know if it should override its builder.

[MariusVanDerWijden]

LGTM