add(scan): Implement SubscribeResults request for scan service

[arya2]

Motivation

We want to be able to subscribe to new scan results to stream to RPC clients.

Closes #8206.

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Solution

• Updates process_messages() function to return a list of subscribed keys and their result senders
• Adds a watch channel for sending subscribed_keys to scan_range() tasks
• Checks if there's a result_sender for a key and sends it new results if there are any

Testing

• Updates test vector for process_messages() to check that it processes SubscribeResults messages correctly
• Adds a test for the ScanService to see that it sends the message correctly
• Adds an acceptance test for checking that the results channel receives new relevant transactions

Review

Anyone can review.

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Follow Up Work

• Refactor ScanTask in zebra-scan #8250
• Add scan_subscribe_results test to CI #8259
• Implement scan grpc method #8256

[arya2]

The acceptance test should pass now, the test was blocking the async executor with a blocking .recv_timeout() call and there weren't any relevant transactions in the height range that was being scanned before the timeout.

[oxarbitrage]

It looks good to me, i made some small suggestions and some questions.