ChainSpec genesis header as SealedHeader

[emhane]

Describe the feature

Make ChainSpec store a SealedHeader instead of genesis header + hash separately

reth/crates/chainspec/src/spec.rs

Lines 223 to 233 in 2ee7748

	/// The hash of the genesis block.
	///
	/// This is either stored at construction time if it is known using [`once_cell_set`], or
	/// computed once on the first access.
	pub genesis_hash: OnceLock<B256>,
	/// The header corresponding to the genesis block.
	///
	/// This is either stored at construction time if it is known using [`once_cell_set`], or
	/// computed once on the first access.
	pub genesis_header: OnceLock<Header>,

Additional context

Ref #14066 (comment)

[DarkLord017]

Hey @emhane , would like to work on this !

[emhane]

how is it going with this one @DarkLord017 ?

[emhane]

taking this over @DarkLord017 to unblock critical path for #13145