feat: support isolated API instances

[marcozabel]

This PR

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.

Related Issues

Fixes #170

Notes

Follow-up Tasks

How to test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.04%. Comparing base (2123274) to head (07aa1f2).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #171      +/-   ##
============================================
+ Coverage     95.19%   97.04%   +1.84%     
- Complexity      227      229       +2     
============================================
  Files            40       41       +1     
  Lines           583      575       -8     
============================================
+ Hits            555      558       +3     
+ Misses           28       17      -11     

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[gemini-code-assist]

Code Review

This pull request implements isolated OpenFeature API instances by changing the OpenFeatureAPI constructor to public and introducing a new OpenFeatureAPIFactory. This allows for independent API instances with their own providers, hooks, and contexts, fulfilling specification requirements. Feedback was provided regarding the unit tests, which currently interact with the global singleton; it is recommended to use multiple isolated instances in these tests to prevent global state pollution and potential test flakiness.