Return the transaction fee in the mempool response

[teor2345]

Motivation

ZIP-401 weights transactions by their transaction fee, so we need to return the transaction fee in successful mempool verification responses.

This Ticket Depends On

remaining_transaction_value needs the UTXOs spent by the block:

• Look up the UTXOs in the transaction verifier, then pass them to verify_transparent_inputs_and_outputs #2440

Consensus Rules

low_fee_penalty is 16000 if the transaction pays a fee less than the conventional fee, otherwise 0

https://zips.z.cash/zip-0401#specification

Design

This function already calculates the transaction fee - it can be easily modified to return it.

zebra/zebra-state/src/service/check/utxo.rs

Lines 224 to 227 in 1ccb2de

	pub fn remaining_transaction_value(
	prepared: &PreparedBlock,
	utxos: &HashMap<transparent::OutPoint, transparent::Utxo>,
	) -> Result<(), ValidateContextError> {

[teor2345]

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[teor2345]

This ticket's dependencies conflict with the NU5 block validation code freeze, so it needs to be done after NU5 testnet activation in sprint 19.

[teor2345]

Unblocked by PR #2849