change(consensus): Allow transactions spending coinbase outputs to have transparent outputs on Regtest

[arya2]

Motivation

We want Zebra's behaviour on Regtest to match zcashd as closely as possible, and zcashd allows transactions spending from coinbase inputs to have transparent outputs on Regtest.

Closes #8478.

Solution

• Adds a should_allow_unshielded_coinbase_spends field to testnet::Parameters
  • Sets it as true on Regtest
• Updates the coinbase_spend_restriction() method to always return OnlyShieldedOutputs on Regtest

Tests

Added a unit test to check that the transaction verifier returns Ok for a transaction that spends coinbase inputs and has transparent outputs.

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.

[arya2]

@mergify update

[mergify]

update

✅ Branch has been successfully updated

[upbqdn]

I think it would be better to change the name of the PR name so that it mentions spending coinbase outputs instead of inputs.