Avoid copying transactions in `get_mergeable_orders`

[MegaRedHand]

See #212 (comment)

When extracting the mergeable orders inside get_mergeable_orders, we're copying each transaction. We should instead modify the execution payload type to store transactions as Bytes, so we can cheaply clone them here.

The clone is inside this to_vec call:

helix/crates/api/src/builder/api.rs

Line 612 in 9d0cad6

transaction: Bytes::from(raw_tx.to_vec()),

The current type of the raw_tx is &VariableList, which comes from inside lh_types::ExecutionPayload. To change this, we'd need to implement our own ExecutionPayload type, changing each transaction to be Bytes instead.

This same approach can be used to avoid cloning blobs from blob transactions:

helix/crates/api/src/builder/api.rs

Lines 706 to 708 in 9d0cad6

	let commitment = block_blobs_bundles.commitments[index].clone();
	let proof = block_blobs_bundles.proofs[index].clone();
	let blob = block_blobs_bundles.blobs[index].clone();