[FEATURE] Implement multi-provider

The mutliprovider needs to be implemented in Python. Instead of being added to the contrib repo, these are now to be added directly into the SDK (similar to the in-memory providers).

See full spec here: https://openfeature.dev/specification/appendix-a/#multi-provider

See implementations in JS, Go.

feat: implement Multi-Provider

Hi! 👋

I've implemented the Multi-Provider in PR #566.

What's included

MultiProvider class - wraps multiple providers into one
FirstMatchStrategy - sequential evaluation (stops at first success)
EvaluationStrategy protocol - for custom strategies
Provider name uniqueness - auto-generates unique names when needed
Parallel initialization - all providers init concurrently
Comprehensive tests - all flag types, fallback behavior, error handling

Example

from openfeature import api
from openfeature.provider import MultiProvider, ProviderEntry

multi = MultiProvider([
    ProviderEntry(primary_provider, name="primary"),
    ProviderEntry(fallback_provider, name="fallback")
])

api.set_provider(multi)

The implementation follows the OpenFeature spec and uses the JS/Go implementations as references.