add(rpc): Adds `getblockheader` RPC method

[arya2]

Motivation

This is needed for block explorer support and has been requested by a mining pool.

Closes #8442. Part of #8446 (always includes height in response when called with verbosity = 2).

Specifications & References

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

Solution

• Adds a getblockheader RPC method like the one in zcashd

Related changes:

• Adds height field to getblock response when called with a height if verbosity == 2

Tests

• Adds a snapshot test for querying by height/hash and with verbose being true/false
• Adds a vectors test

PR Author's Checklist

• [x] The PR name will make sense to users.
• [x] The solution is tested.
• [x] The documentation is up to date.
• [x] The PR has a priority label.

PR Reviewer's Checklist

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

[arya2]

zcash-rpc-diff util output with verbose = true:

```console { "hash": "000000002c2063bab0acae547bab7e0309db8e1b7003e9c01eb423c0166d5ac9", - "confirmations": 2683651, + "confirmations": 169203, "height": 10000, "version": 4, "merkleroot": "e093171fc092c4bc0598c6d475489f892b177e3c6139ca680b064787093e2d1e", - "finalsaplingroot": "fbc2f4300c01f0b7820d00e3347c8da4ee614674376cbc45359daa54f9b5493e", + "finalsaplingroot": "0000000000000000000000000000000000000000000000000000000000000000", "time": 1479096667, "nonce": "000000000000000000000000000004cc00000000000000000000000000000000", + "solution": "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", "bits": "1d01bf9d", - "difficulty": "00000001bf9d0000000000000000000000000000000000000000000000000000", + "difficulty": 299851.4066097095, + "chainwork": "00000000000000000000000000000000000000000000000000000a13fdd5bc69", "previousblockhash": "000000001298a3685fbc493751ff2309a9bcef7cefad4565642bafb43f71b4c6", "nextblockhash": "00000000980d28a4bd926c4c553f7eaa5b29d3a78e8423f3863b72edd258839e" } ```

It matches exactly when verbose is false, but the difficulty and finalsaplingroot fields aren't matching. It's possible the finalsaplingroot field will match after Sapling activation (my zcashd is still syncing). zcashd also includes solution and chainwork fields, solution should be trivial to add, chainwork may be a little more complicated.

Update:

The finalsaplingroot does match after Sapling activation, though it was reversed.

Now it's just the difficulty field that's different, and the solution and chainwork fields that are missing.

Update 2:

The RPC is still missing the chainwork field but otherwise matches the zcashd output exactly, including while the Sapling note commitment tree is empty.

The chainwork field is confirmed to be the cumulative work in the chain, which Zebra doesn't store in the finalized state. It's also undocumented so we can probably leave it until/unless someone requests it. It could also still be calculated by the caller by summing up the work for the chain up to the requested block.

[oxarbitrage]

The chainwork field is confirmed to be the cumulative work in the chain, which Zebra doesn't store in the finalized state. It's also undocumented so we can probably leave it until/unless someone requests it. It could also still be calculated by the caller by summing up the work for the chain up to the requested block.

The chainwork field was mentioned in the last Zcashd deprecation meeting and as it is an undocumented field it should be safe to ignore it in Zebra.

[oxarbitrage]

The failing test seems to be unrelated however it will be good to check that out in another issue/context.

[mpguerra]

@oxarbitrage can we get a final review here if it's all ok?

[gustavovalverde]

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[mpguerra]

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[gustavovalverde]

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