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

### 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