fix: Enable faster feature loading

[zachaysan]

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Changes

In order to support faster loading on the feature page an alteration was done to the feature response, if the environment id is present, then the FeatureState that matches the correct environment and has the proper is_live attributes is set as the feature state for the feature.

I tried to follow the featurestates associated code, although the inclusion of the identity serializer didn't seem necessary for this endpoint so I used a slightly slimmer serializer.

Since using a queryset to slim the relationship from many-to-one to one-to-one wasn't possible, I took a page from the viewset and associated the individual models manually with an IN clause then kept the collection for association in the serializer.

If the environment is not present then all that is different is that the value for the feature_state part of the response is set to None.

How did you test this code?

None of the existing test methods really suited this test so I made a new one and checked the multivariate part of the response as well as basic functioning of the endpoint as well.

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[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: api/features/serializers.py

Function	Unhandled Issue
validate	KeyError: 'percentage_allocation' /api/v1/environ... Event Count: 68

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.91%. Comparing base (c637e86) to head (d91199d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3550   +/-   ##
=======================================
  Coverage   95.90%   95.91%           
=======================================
  Files        1102     1102           
  Lines       34706    34782   +76     
=======================================
+ Hits        33286    33362   +76     
  Misses       1420     1420           

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