Allow setting a minimum gas used value to determine whether or not to use a block supplied by the builder API

In a similar spirit to https://github.com/flashbots/mev-boost/issues/273

I think users should be able to express an opinion about the fullness of a block. Currently, block builders will often leave transactions out of a block when volatility is high. The reasons for this are not fully known, however what is know is that blocks have a lower gas used than would otherwise be the case.

One of the purported benefits of delegating block building is that the block builder should be able to fill the block with more transactions than a solo validator.

This would allow solo validators to express their opinions.

The issue names no files, tests, or entry points to begin with. First map how builder API blocks are evaluated and where validator configuration is handled, then define the minimum-gas setting and its acceptance behavior before implementing tests and the change.

Written by the indexing model from the issue text.