Make gas_limit setting an option

[mattsse]

Describe the feature

we need to distinguish between default and custom setting, hence we need to make this value an option

##TODO

convert this into an option:

reth/crates/node/core/src/args/payload_builder.rs

Lines 19 to 21 in b1a1da1

	/// Target gas limit for built blocks.
	#[arg(long = "builder.gaslimit", default_value_t = ETHEREUM_BLOCK_GAS_LIMIT_36M, value_name = "GAS_LIMIT")]
	pub gas_limit: u64,

and this as well

reth/crates/node/core/src/cli/config.rs

Line 28 in b1a1da1

fn gas_limit(&self) -> u64;

this can use unwrap_or

reth/crates/ethereum/node/src/payload.rs

Line 52 in b1a1da1

EthereumBuilderConfig::new().with_gas_limit(conf.gas_limit()),

with

reth/crates/node/core/src/args/payload_builder.rs

Line 44 in b1a1da1

gas_limit: ETHEREUM_BLOCK_GAS_LIMIT_36M,

Additional context

No response

[VeerChaurasia]

will do