ZIP-213: Validate Shielded Coinbase Outputs like other shielded outputs

[teor2345]

Dependencies

This ticket depends on some parts of pre-Heartwood transaction validation, see #801.

Motivation

When we implement transaction validation, we should make sure we implement the Shielded Coinbase rules from Heartwood ZIP-213:
https://zips.z.cash/zip-0213#specification

Solution

Make sure Zebra verifies the ZIP-213 consensus rules:

• The consensus rules applied to valueBalance, vShieldedOutput, and bindingSig in non-coinbase transactions MUST also be applied to coinbase transactions.
  • Note: this rule originally applied to Sapling, but we assume it also applies to Orchard

The following consensus rules are implemented in coinbase_tx_no_prevout_joinsplit_spend:

• A coinbase transaction MUST NOT have any transparent inputs, JoinSplit descriptions, or Spend descriptions.
• In a version 5 coinbase transaction, the enableSpendsOrchard flag MUST be 0.

The Sapling and Orchard note consensus rules will be implemented in #2362.

The following consensus rule will be implemented in #1970:

• Only transparent outputs in coinbase transactions are subject to the existing restrictions on spending coinbase funds

Existing Work

The draft implementation PR is:
#1190