Add support for getblockhash RPC call

[mpguerra]

Motivation

Mining pools use the getblockhash RPC method.

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

Requirements

Testing

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

[mpguerra]

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