Add featureEnabled to variant response

[FredrikOseberg]

We've started the path to add featureEnabled to the variant response. This is useful when you want to differentiate between a feature flag that is disabled and a feature flag that is enabled but has no variants.

Reference implementation in python:

• feat: add is_feature_enabled to variant response unleash-client-python#285

Specification tests:

• feat: add feature_enabled property to variants client-specification#70

If you want to contribute to Unleash, this is a good place to start.