[FEATURE] Implement Multiprovider

[toddbaert]

This issue proposes to add support for multiple feature flag providers within the OpenFeature SDK.

Motivation

Supporting multi-provider functionality would allow users to integrate and delegate feature flag evaluations across several providers in the same application. This improves flexibility and enables complex use cases where different providers might be required for different environments or flag types.

Reference

For more details on the multi-provider concept in OpenFeature, please see the official specification:
https://openfeature.dev/specification/appendix-a#multi-provider

See implementations in JS, Go.