Implement isolated API instances across SDKs

[toddbaert]

Context

The spec now supports isolated API instances; independent, non-singleton instances of the OpenFeature API, each with their own state (providers, context, hooks, events). This enables use cases like micro-frontends, dependency injection, and testing.

• Spec PR: feat: add isolated instance #368
• Original issue: Considering alternatives to the OpenFeature global singleton #359

Relevant spec sections

• Section 1.8 - Isolated API Instances
• Glossary - Isolated API Instance

Requirements summary

Requirement	Level	Description
1.8.1	MUST	Provide a factory function that creates independent API instances with their own state
1.8.2	MUST	Isolated instances conform to the same API contract as the singleton
1.8.3	SHOULD	Factory function lives in a distinct module for discoverability
1.8.4	SHOULD NOT	A provider instance should not be bound to multiple API instances simultaneously

SDK implementation tracking

Child issues:

• Java: Support isolated API instances java-sdk#1925
• Go: Support isolated API instances go-sdk#486
• .NET: Support isolated API instances dotnet-sdk#734
• JS: Support isolated API instances js-sdk#1373
• PHP: Support isolated API instances php-sdk#170
• Python: Support isolated API instances python-sdk#584
• Swift: Support isolated API instances swift-sdk#115
• Kotlin: Support isolated API instances kotlin-sdk#229