[builder API] Make gas limit configurable

[realbigsean]

Description

Currently we are hard-coding , this should be configurable on a per-validator basis. We will likely want to track this in the validator definitions file, and allow users to update it via the HTTP API.

[mrabino1]

Interesting.. so the 30MM gas limit would be variable.. I would hope that we have a parameter to allow for the default to be 30MM

[realbigsean]

There's some discussion around it here: ethereum/builder-specs#17

But yes, I think we'd keep 30MM as a default, just want to make sure it's possible for validators to set it if they wish

[michaelsproul]

I've opened a PR to add a standard HTTP API for the gas limit too: ethereum/keymanager-APIs#39

I'll open a Lighthouse issue for its implementation if the feedback is favourable