feat: Grafana feature health provider

[khvn26]

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Changes

This PR implements the Grafana (or Prometheus) health provider. Additionally, the feature health framework is refactored:

• Feature health events can be associated with an external ID. When retrieving events for UI, latest events are now selected per project per feature per environment per provider per external ID. This is done to allow tracking multiple different alerts targeting the same feature.
• FeatureHealthEvent.created_at can now be set to arbitrary datetime. This is done so we can use datetimes provided by alertmanager.
• FeatureHealthEvent.reason is now a JSONField with predetermined schema common for all providers. This enables supplying arbitrary number of text and URL data with the feature health event.
• Feature health providers are now allowed to produce multiple feature health events from one webhook call.
• A service layer is added to individual providers to potentially allow for more than simple mapping logic.

How did you test this code?

Added integration tests:

• For a basic scenario.
• For a case when different alerts hit the same webhook.

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Docker builds report

Image	Build Status	Security report
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ghcr.io/flagsmith/flagsmith-api-test:pr-5098	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-5098	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-5098	Finished ✅	Results ✅
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.46%. Comparing base (5de0b42) to head (7ffe3b6).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5098      +/-   ##
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+ Coverage   97.45%   97.46%   +0.01%     
==========================================
  Files        1216     1224       +8     
  Lines       42368    42537     +169     
==========================================
+ Hits        41289    41458     +169     
  Misses       1079     1079              

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[matthewelwell]

2. We should ideally make sure that this field isn't needed in the 'list' endpoint as I believe we'll have to defer it in the queryset, resulting in an N+1 problem.

@khvn26 this is an example of the logic I'm referring to with my comment here

https://github.com/Flagsmith/flagsmith/blob/main/api/permissions/permission_service.py#L147-L152

[khvn26]

@matthewelwell Reverted to TextField to avoid JSONField usage, and removed the squashed migrations.