[Feature] Use lazy hashing for Signed

[mattsse]

Component

consensus, eips, genesis

Describe the feature you would like

currently the signed type mandates a hash:

alloy/crates/consensus/src/signed.rs

Lines 15 to 16 in dc3a7fe

	#[doc(alias = "tx_hash", alias = "transaction_hash")]
	hash: B256,

ideally we can relax this and instead compute the hash on demand if missing via a OnceLock

https://github.com/paradigmxyz/reth/blob/36807928311a91db559db228c3d25615152b4c8b/crates/ethereum/primitives/src/transaction.rs#L298-L300

https://github.com/paradigmxyz/reth/blob/36807928311a91db559db228c3d25615152b4c8b/crates/ethereum/primitives/src/transaction.rs#L355-L359

the signed value is currently just a type T, to compute the hash on demand we'd either need to introduce a new trait (basically Sealable) or we require that T: Transaction
I think we could make T: Transaction because this type is already intended for transactions:

alloy/crates/consensus/src/signed.rs

Lines 6 to 9 in dc3a7fe

	/// A transaction with a signature and hash seal.
	#[derive(Clone, Copy, Debug, PartialEq, Eq)]
	#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
	pub struct Signed<T, Sig = Signature> {

TODO

• replace B256 with OnceLock

Additional context

No response