[Feature Store] Making features/entities optional in feature-set spec

[theSaarco]

The code in FeatureSetSpec had features and entities as mandatory lists. This means that if a user does feature_set.save() on an object without them, it will cause failures. This is not the expected behavior.
As a side effect, once they became optional, had to change the way feature_set.dict() behaves and add exclude_none=True, because otherwise the dictionary created would have (for example) {"entities": None}, and various get calls such as entities = feature_set_spec.pop("entities", []) would return None rather than the expected [] since the default value is only used when the key does not exist in the dictionary, not when it is None. Using exclude_none fixes this behaviour.