add(scan): Implement ClearResults ScanService request

[arya2]

Motivation

This PR implements the ClearResults request for ScanService to delete the results for a key without removing the key itself.

Closes #8207.

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Solution

• Renames delete_sapling_results to delete_sapling_keys
• Adds a delete_sapling_results that adds back an empty entry after deleting the results for a sapling key
• Calls the new storage method from the ScanService::call() match arm for Request::ClearResults

Related cleanups:

• Moves check for max number of keys to a request method

Testing

• Tests that sapling_delete_results() deletes results but leaves an empty entry with the key
• Tests that the scan service ClearResults request does the same

Review

Anyone can review.

Reviewer Checklist

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• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
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Follow Up Work

Test that the scan service responds with an error for requests with an invalid number of keys.

[oxarbitrage]

Looks good to me, left a few minor comments.