Demo client-side providers for playground

[toddbaert]

We should create some demo providers in the playground, as we did initially with the server spec.

I've done a few of these on this branch:

• Flagsmith
• LaunchDarkly
• Split

I plan on doing Cloudbees FM and flagd also.

[toddbaert]

Completed with https://github.com/open-feature/playground/pull/131