Add flagd UI

[maindotmarcell]

Changes

We're adding a user interface to the demo, through which users of the demo can update the Flagd feature flag configuration file in a more user friendly way.

Finished changes

• Created the flagd-ui directory
  • Initialized a Next.js application
  • Created a basic view in which users can set flags in a user friendly way
  • Created an advanced view in which users can interact with the json directly from the app (includes json schema checking to ensure that the file stays valid)
  • Created a navigation bar to be able to navigate easily between the views
  • Instrumented with OpenTelemetry automatic instrumentation
  • Added Dockerfile
• Made necessary changes to docker-compose.yaml (config file is mounted as a volume)
• Made changes to the frontend-proxy configuration so this UI is accessible at :8080/feature

Remaining changes

• Before merging this PR
  • Make necessary updates to the documentation
• After merging this PR
  • Create updates for helm-charts (as discussed in the SIG call) for k8s deployment

Suggested roadmap for future contributions

• Support for fractional targeting
• Solution for compatibility with flagd sidecar containers for general usage (outside of the demo)

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: maindotmarcell / name: Marcell Münnich (e33d518)
• ✅ login: bornav (55cb083)

[beeme1mr]

Hey @maindotmarcell, I'm Mike from the OpenFeature team. Thanks for getting this started! Please let me know if you need our support.

By the way, I've been experimenting with a flagd compatible rule builder React component. Perhaps I could contribute to that once the initial version is live.

[maindotmarcell]

Hi @beeme1mr, sounds awesome! Thanks for letting me know, we could definitely add something like a rule builder to this eventually :)

[mviitane]

Hi @maindotmarcell, there are some failing checks related to markdown-lint, sanity, and license. Could you fix those so we can proceed with the PR?