feat: add Test traits for Header and Block

[mattsse]

towards #13626

why we need these

currently the SealedBlock has some weird deref_mut

reth/crates/primitives/src/block.rs

Lines 169 to 170 in f7ea2f4

	#[deref_mut]
	header: SealedHeader<H>,

that deref_muts to these testing helpers:

reth/crates/primitives-traits/src/header/sealed.rs

Lines 140 to 141 in f7ea2f4

	#[cfg(any(test, feature = "test-utils"))]
	impl SealedHeader {

with #13735 we won't have access to this easily because we don't have SealedHeader, so we could either do it via these traits or we duplicate all these setters for SealedBlock<alloy::Block>

imo these traits are easier to manage

[Rjected]

should work for test utils