Allow configuring a desired gas limit on the block builder

[MegaRedHand]

Related to #2526 (this one is for L1)

Currently, we have a hardcoded desired gas limit of 30M on the block builder:

ethrex/crates/blockchain/payload.rs

Line 182 in 9d61e15

const DEFAULT_BUILDER_GAS_CEIL: u64 = 30_000_000;

We should instead have this as a configurable value somewhere.