zebra: Add support for `getblocksubsidy` RPC call

Motivation

Mining pools use the getblocksubsidy RPC calls.

Mining Pool Usage

s-nomp calls getblocksubsidy from this code:

• https://github.com/s-nomp/node-stratum-pool/blob/d86ae73f8ff968d9355bb61aac05e0ebef36ccb5/lib/pool.js#L604

Requirements

https://zcash.github.io/rpc/getblocksubsidy.html

Height argument

• is optional.
• is a number (current zebra functions that have height as argument are a string).

Usage

This RPC is called for the next block height to be mined, before that block is created or validated by the node. The expected response is the subsidy amounts. Transaction fees are not included.

This RPC does not need to support founders reward heights, because all those blocks are mined already.

Design

We need the fixed miner reward and funding stream values for future heights.

Fields:

• fundingstreams: The required amount sent to each funding stream address
  • the specification is a hard-coded value
• miner: the required amount of the miner reward
• foundersreward:
  • The required amount sent to a founders rewards address
  • always has a fixed value of zero, because the RPC is not called for existing blocks

We can get the funding stream recipient addresses and amounts fromfunding_stream_values(): https://github.com/ZcashFoundation/zebra/blob/dc9da24c8b6cee5c05315f0d77bee52613f75fd3/zebra-consensus/src/block/subsidy/funding_streams.rs#L21-L28

We can get the fixed miner reward from zebra_consensus::subsidy as well.

Error Handling

If the RPC is called for a founders reward block, just return an error.

Testing

• Snapshot the output of the RPC
• Manually compare the zcashd and zebrad RPC output using https://github.com/ZcashFoundation/zebra/blob/main/zebra-utils/zcash-rpc-diff

Follow-Up Work

Testing:

• Test calling the RPC from the mining pool software (in ticket #5934)