Implement Multi-Provider

[lukas-reining]

The Multi-Provider allows you to use multiple underlying providers as sources of flag data for the OpenFeature server SDK. When a flag is being evaluated, the Multi-Provider will consult each underlying provider it is managing in order to determine the final result. Different evaluation strategies can be defined to control which providers get evaluated and which result is used.

The Multi-provider is specified in the appendix of the OpenFeature spec: https://openfeature.dev/specification/appendix-a#multi-provider

There already is an implementation for JS: https://github.com/open-feature/js-sdk-contrib/tree/main/libs/providers/multi-provider