Add feature flag service

[Blinkuu]

To make quickpizza more useful for demoing k6 x Tempo, we should add a feature flag service and allow fault simulation.

[markjmeier]

+1, I think quickpizza stands to become a really good demo app. I'd like to use it for the demo block that SEs are giving. So building it out to help us demo various product functionality, is a good idea.

[dgzlopes]

Related to #6 --> They should be under a feature flag.

IMO: It would be great if these flags could be changed from the frontend on the fly!