Add v5 transaction stubs

[teor2345]

Is your feature request related to a problem? Please describe

Network Upgrade 5 introduces a new transaction version. Zebra needs to support that transaction version.

The draft ZIP isn't available yet:

• ticket: [ZIP 225] Version 5 Transaction Format zcash/zips#440
• ZIP: https://github.com/zcash/zips/blob/master/zip-0225.rst

But there is a draft spec for the transaction format at: https://raw.githubusercontent.com/daira/zips/orchard-circuit/protocol/orchard.pdf

So for now we just want to create a placeholder enum variant.

Describe the solution you'd like

Transaction variant

• Add a V5 { bytes: Vec<u8>; } variant to the Transaction enum: https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/transaction.rs#L47

Serialization

• Serialize and deserialize raw V5 transactions to bytes: https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/transaction/serialize.rs#L71
  • Add a proptest v5_strategy to test serialization/deserialization: https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/transaction/arbitrary.rs#L75

Suggestion - just do something simple like:

1. choose a random length between 0 and 20
2. fill it with random bytes

We'll rewrite it properly later.

Validation

• Reject V5 transactions during transaction validation: https://github.com/ZcashFoundation/zebra/blob/main/zebra-consensus/src/transaction.rs#L135
  • test that the transaction validator rejects v5 transactions

Placeholders

• If a V5 transaction is used anywhere else, panic using unimplemented!("v5 transaction")

Describe alternatives you've considered

We could try to implement the whole v5 transaction format in the same PR, but that's a lot of work.

Additional context

This is just a stub that lets us start work on v5 transactions.

Follow Up

Once #1797 merges:

• Reject V5 transactions during transaction validation if the network upgrade is Canopy or earlier
• If the network upgrade is NU5 or later, panic using unimplemented!("v5 transaction")
• test that the transaction validator rejects v5 transactions before NU5, and panics after NU5 (using #[should_panic])

Suggestion: add an allows_v5_transactions() -> bool method on NetworkUpgrade

Implement the v5 transaction format based on the draft spec: https://raw.githubusercontent.com/daira/zips/orchard-circuit/protocol/orchard.pdf