Create a scan_block function to use across scanning tasks

[upbqdn]

Motivation

Close #7947.

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Solution

I wrapped the original scanning function so it:

• takes:
  • Zebra's blocks;
  • Zebra's network type;
  • keys without accounts;
• doesn't take:
  • Sapling nullifiers;
  • prior block metadata.

Testing

I updated two existing tests. Two more use the original scanning function, but they're better off with that one.

Review

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• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
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[oxarbitrage]

Looks really good, it simplifies a lot of things and clarifies what is really needed and what can be left behind.

It is nice to see that you added the function into the test module first, i think we should move the "production code" from there after #7989 merges into the scan.rs file.

I made a few optional suggestions and comments, well done!

[teor2345]

This seems good enough to merge, we can do more tests and move code around later?