fix spec metadata structure - #417

The FlashblockMetadata described in the spec is different from the implementation. I assumed the implementation is correct, and have updated the spec to reflect the implementation.

Transaction receipts included in the flashblocks metadata are serialized from the following enum defined here.

pub enum OpReceipt {
 /// Legacy receipt
 Legacy(Receipt),
 /// EIP-2930 receipt
 Eip2930(Receipt),
 /// EIP-1559 receipt
 Eip1559(Receipt),
 /// EIP-7702 receipt
 Eip7702(Receipt),
 /// Deposit receipt
 Deposit(OpDepositReceipt),
}

This does not translate to Python succinctly so I opted to show it in Rust. I think that it makes sense to update the spec to show all of the types in Rust to make it easy to translate to the code stored in the repo, and I'm happy to make that change if it's agreed.