change(rpc): Implement the z_getsubtreesbyindex RPC

[teor2345]

Motivation

This PR implements the z_getsubtreesbyindex RPC method and return type.

It needs data from PR #7350 for testing, but it can merge without that PR and we can write tests later.

Close #6954.

Specifications

See https://github.com/zcash/zcash/pull/6677/files#diff-decae4be02fb8a47ab4557fe74a9cb853bdfa3ec0fa1b515c0a1e5de91f4ad0bR1458-R1475

The specs for this RPC aren't on the RPC spec site yet.

Complex Code or Requirements

This RPC must return the same data and format as zcashd.

The data for this RPC must be added to the database from the non-finalized tip backwards (or all at the same time), see ticket #7407 for details.

Solution

Implement the RPC method, arguments, and return type

• Validate the pool argument
• Match zcashd's behaviour of returning an empty list if the start_index is not available
• Don't error if start_index + limit would overflow, matching zcashd's behaviour

Testing

We'll split tests into separate tickets, as requested here:
#6954 (comment)

New RPC methods will need:

• new snapshot tests
• new integration tests
• manual zcash-rpc-diff test results in a comment on the PR, to make sure we match zcashd

And possibly:

• new unit tests

Review

This PR is ready for review, we will write tests in other tickets.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?

[teor2345]

Temporary network error on a draft PR, doesn't need fixing or even restarting:

Error: buildx failed with: ERROR: failed to solve: error writing layer blob: failed to authorize: failed to fetch oauth token: Get "https://us-docker.pkg.dev/v2/token?scope=repository%3Azfnd-dev-zebra%2Fzebra-caching%2Fzebrad-test%3Apull%2Cpush&service=us-docker.pkg.dev": read tcp 10.1.0.161:46942->142.250.115.82:443: read: connection reset by peer

https://github.com/ZcashFoundation/zebra/actions/runs/6031782300/job/16365993221?pr=7436#step:10:1811

[upbqdn]

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