engine: add targetGasLimit to PayloadAttributesV4

[nflaig]

Adds targetGasLimit to PayloadAttributesV4 so that the CL can pass the target gas limit to the EL on engine_forkchoiceUpdatedV4, instead of relying on a static flag on the EL side for local building.

This is useful for several reasons

1. it unifies where users have to configure their target gas limit
2. it allows to configure the gas limit per validator/pubkey, similar to the fee recipient
3. allows us to use vanilla ELs as builders, see comment

Based on #608 (credit to @mcdee)

[nerolation]

LGTM!

Just to note, this means the EL has to then determine the block.gas_limit based on the target_gas_limit, e.g.:

max_delta = parent_gas_limit // 1024

block.gas_limit = max(
    parent_gas_limit - max_delta,
    min(
        target_gas_limit,
        parent_gas_limit + max_delta,
    ),
)

[nflaig]

Just to note, this means the EL has to then determine the block.gas_limit based on the target_gas_limit, e.g.:

should the engine spec have more details about this? from the EL side nothing should change, just the source from where they get the target gas limit changes

[mkalinin]

The spec change looks good to me 👍 Is there a consensus among CL and EL devs on the acceptance of this change?

[nflaig]

Is there a consensus among CL and EL devs on the acceptance of this change?

we've discussed this on ACDT 79 yesterday, there was no opposition to this but we should probably get approvals before merging this change

[barnabasbusa]

I think this must be bundled together with the removal of gas limit target flags on the EL side. Otherwise the expected behaviour might be unknown. If we merge this in, it would be good for EL teams to start signalling that the EL side gas limit flag will be deprecated post glamsterdam, and probably following the fork the flag should just go away completely.