feat(config): Allow to add keys to be scanned by the zebra-scan crate to config

[oxarbitrage]

Motivation

We want the user to add sapling keys to the config file so Zebra can scan the blockchain with them.

Close #7941

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Solution

Add a config type to the zebra-scan crate with a sapling_keys_to_scan field of type IndexMap<String, u32> where the string is the key and the number the birthday height.

This field is used as:

[scan.sapling_keys_to_scan]
"somekey" = 1
"someotherkey" = 100000

The configuration is behind a zebra-scan rust feature so build should be done as follows to see the new section:

$ cargo build --features="zebra-scan"

Testing

This PR has been tested manually.

Review

This is a very basic version introducing the less amount of code possible, open to make changes.

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?
• Are all the PR blockers dealt with?
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Follow Up Work

[teor2345]

Looks great!

Adding zebra-scan as a dependency in production zebrad builds might be a blocker. Everything else is optional, I've tried to move as much work out of the MVP and onto the tracking issue as possible.

[teor2345]

Looks good to me, thanks for the quick changes

[teor2345]

CI failed due to #7898:
https://github.com/ZcashFoundation/zebra/actions/runs/6884108042/job/18726073944?pr=7949#step:5:199

So I'll restart it.