feat(api): add isolated API instances

[marcozabel]

This PR introduces support for creating isolated OpenFeature API instances, each with their own providers, hooks, context, and event handling - enabling multi-tenant or side-by-side usage without shared global state.

Fixes #584

[codecov]

Codecov Report

❌ Patch coverage is 97.85523% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.25%. Comparing base (7d9229f) to head (ffcbf36).

Files with missing lines	Patch %	Lines
openfeature/_event_support.py	85.71%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
- Coverage   98.33%   98.25%   -0.09%     
==========================================
  Files          45       48       +3     
  Lines        2157     2458     +301     
==========================================
+ Hits         2121     2415     +294     
- Misses         36       43       +7     

Flag	Coverage Δ
unittests	98.25% <97.85%> (-0.09%)	⬇️

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[gemini-code-assist]

Code Review

This pull request introduces isolated OpenFeature API instances, enabling multiple independent configurations within a single application as per specification requirement 1.8. The core logic refactors global state into an OpenFeatureAPI class, with the existing global singleton now delegating to a default instance to maintain backward compatibility. Key changes include the introduction of an EventSupport class, updates to the OpenFeatureClient to reference specific API instances, and the addition of a new openfeature.isolated module. Review feedback recommends improving encapsulation by adding public methods to OpenFeatureAPI for retrieving providers and their status, rather than having the client access protected registry attributes directly.