Add feature enabled property to variant response

[FredrikOseberg]

Is there an existing issue for this?

• [X] I have searched the existing issues

Describe the new feature

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:

• https://github.com/Unleash/unleash-client-python/pull/285

Specification tests:

• https://github.com/Unleash/client-specification/pull/70

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