feat(feature-flags): use flipt as feature flag service

[nimdanitro]

Details:

• use flipt.sitrep.ch as feature flag service, admin access is possible with f-eld account, but instance is readonly.
• properly setup and use flipt with openfeatureflags abstractions
• setup CICD tasks to build and push feature flag config to allow gitops workflow for featureflags
• ui/features.yaml is authoritative for the namespace sitrep-ui and all feature flags for frontend should be tracked here.

[lukastresch]

Thanks for your work @nimdanitro! 💪🏼