Manually test the `getblocktemplate` RPC creates valid blocks

[oxarbitrage]

Our final goal is to support mining pools to use zebra for their operations. This requires multiple rpc methods to be present.

There is a discussion open to exchange ideas on this.

Details

From #5804 (comment)

I think we can do testing using a block proposal, which is a block without a proof of work.

Here's how we could do that:

• Finish all the required getblocktemplate RPC fields
• Create a block header from the RPC fields, with a zeroed-out equihash solution, and serialize it
• Create the block transactions by combining all the coinbasetxn and transaction data fields
• Send that block to zcashd using `getblocktemplate '{ "mode": "proposal", "data": "(hex data)" }'

zcashd has proposal support, but it's not documented. Here's the spec:
https://en.bitcoin.it/wiki/BIP_0023#Block_Proposal

Related Work

• Manually test that each mining RPC works correctly #5686
• Manually test the getblocktemplate RPC creates valid blocks #5803
• Manually test that Zcash mining pools can use Zebra to mine blocks on testnet #5934
• Manually test that the Zebra and zcashd mempools are similar #5935

[teor2345]

@oxarbitrage please tag your tickets with A-rpc, so we can find them when we're looking for RPC work.