add(scan): Create a tower Service in zebra-scan

[arya2]

Motivation

This PR adds a ScanService struct to zebra-scan to be used by RPC methods in zebra-grpc.

Closes #8182.

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Solution

• Defines ScanService/ScanTask structs and ScanTaskCommand enum
• Defines Request and Response enums in zebra-node-services
• Implements tower::Service for ScanService
• Adds an instance of Storage and ScanTask to fields on ScanService
• Checks for panics in poll_ready() method
• Adds some possible Request variants and match arms in call() method

Review

Anyone can review.

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Follow Up Work

Implement match arms for Request variants

[upbqdn]

All looks great.

[oxarbitrage]

This looks good to me, i think we should add at least 1 test of the service but we can do it somewhere else. I will use the service, add a new request/response in the context of #8178 and will add a test there.