Return identity_feature_state in the /projects/{project_id}/features?identity=id endpoint

[kyle-ssg]

Is your feature request related to a problem?

The frontend chains a lot of endpoints together in order to display identity overrides which is especially problematic where we have pages of feaures.

Describe the solution you'd like.

/projects/26448/features?environment=id returns environment_feature_state, we should have for /projects/26448/features?identity=id return identity_feature_state

Describe alternatives you've considered

Keep lots of dead code

Additional context

This will help remove a lot of old redux / javascript code