Handle missing `feature_segment` on version create

[sentry]

Currently, the API returns a 500 error response when the feature_segment attribute is missing in the POST JSON. We should review

1. if the FE is sending this, or if this occurrence was manually triggered against our API
2. a better response to communicate the error to the client

Further details from Sentry:

Sentry Issue: FLAGSMITH-API-5PY

KeyError: 'feature_segment'
(11 additional frame(s) were not displayed)
...
  File "features/versioning/views.py", line 151, in perform_create
    serializer.save(
  File "features/versioning/serializers.py", line 174, in create
    self._validate_segment_override_limit(environment)
  File "features/versioning/serializers.py", line 323, in _validate_segment_override_limit
    segment_ids_to_create_overrides=self.segment_ids_to_create_overrides,
  File "features/versioning/serializers.py", line 164, in segment_ids_to_create_overrides
    if fs["feature_segment"] is not None