Adds days since install calculation to the SDK

[tarikeshaq]

Part of https://mozilla-hub.atlassian.net/browse/SDK-384

Some important TODOs before this can land:

• Add tests, for the changed behaviour and for targeting.. Will probably need to write an integration test to validate this
• Add changelog
• modify swift/kotlin code to pass in stub value for installation_date, then address the actual values on another PR

[tarikeshaq]

Added this logic here because it happens off the main thread... Not sure if it's the best place for it

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (nimbus-targeting-object@f739139). Click here to learn what that means.
The diff coverage is 87.71%.

❗ Current head c678953 differs from pull request most recent head 3e7469e. Consider uploading reports for the commit 3e7469e to get more accurate results

@@                    Coverage Diff                     @@
##             nimbus-targeting-object    #4491   +/-   ##
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  Coverage                           ?   76.71%           
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  Files                              ?       48           
  Lines                              ?     4496           
  Branches                           ?        0           
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  Hits                               ?     3449           
  Misses                             ?     1047           
  Partials                           ?        0           

Impacted Files	Coverage Δ
components/nimbus/src/error.rs	11.76% <0.00%> (ø)
components/nimbus/src/persistence.rs	95.12% <ø> (ø)
components/nimbus/src/lib.rs	72.04% <82.73%> (ø)
components/nimbus/src/enrollment.rs	96.66% <97.61%> (ø)
components/nimbus/src/evaluator.rs	98.33% <100.00%> (ø)
...nts/nimbus/tests/test_time_targeting_attributes.rs	100.00% <100.00%> (ø)

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[tarikeshaq]

Huh https://app.circleci.com/pipelines/github/mozilla/application-services/28408/workflows/235fedb9-adfe-4dc6-92a8-2c1980dde3e7/jobs/116169 is very weird... It's a test failure in places.....

[jhugman]

Main changes needed:

• make AppContext immutable again.
• make a new struct with app_directory_path: string and days_since_install: i32? and pass it in as part of a Nimbus constructor.
• calculate install_date and last_update_date and store them in the database.
• calculate days_since_update and days_since_install from the stored dates and now().
• Add a TimeBasedAttributes to TargetingAttributes with the days_since_update and days_since_install.

[tarikeshaq]

Added tests, not 100% happy with how they look, but they test both the targeting and the fallback (it doesn't test that the actual fallback works, but it tests that we end up doing the fallback when we need to) I'll do a second go through the tests to see if I can make them better or cover more logic, but opening this up for more reviews!

[jhugman]

This is looking lovely. There are a few bits I was confused about which you should address/answer/comment, but I don't think I should hold you up in another review cycle.

Good job!