feat: Add multi-provider support

[askpt]

This PR

This pull request introduces the experimental "Multi-Provider" feature to the OpenFeature library, enabling simultaneous use of multiple feature flag providers with configurable evaluation strategies. The changes include updates to documentation, new classes and methods to support multi-provider functionality, and a sample implementation in AspNetCore. Below is a summary of the most important changes grouped by theme.

Documentation Updates:

• Added "Multi-Provider" to the feature list in README.md, marking it as experimental.
• Included a detailed section in README.md explaining the usage, evaluation strategies, modes, and limitations of the Multi-Provider feature.

Sample Implementation:

• Updated samples/AspNetCore/Program.cs to demonstrate Multi-Provider usage, including flag evaluation and error handling. [1] [2]

Core Multi-Provider Functionality:

• Introduced the MultiProvider class in src/OpenFeature/Providers/MultiProvider/MultiProvider.cs, implementing support for multiple providers, evaluation strategies, initialization, and shutdown logic.
• Added the ProviderEntry class to represent individual providers in the Multi-Provider configuration.

Supporting Models and Extensions:

• Created ProviderStatus and RegisteredProvider models to track provider states and metadata. [1] [2]
• Added ProviderExtensions to handle evaluation logic for individual providers.

Related Issues

Fixes #487

Notes

I didn't do proper testing for Hooks and Events. This PR is already quite big, and I would appreciate some feedback on the overall design.

Follow-up Tasks

• Add Hooks and Events support for Multi-Provider.
• Add Dependency Injection support.

How to test

Open the samples app and enjoy!

[codecov]

Codecov Report

❌ Patch coverage is 95.82310% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.67%. Comparing base (417f3fe) to head (bfeab28).
⚠️ Report is 154 commits behind head on main.

Files with missing lines	Patch %	Lines
...enFeature/Providers/MultiProvider/MultiProvider.cs	95.91%	5 Missing and 3 partials ⚠️
...MultiProvider/Strategies/BaseEvaluationStrategy.cs	85.36%	2 Missing and 4 partials ⚠️
...ture/Providers/MultiProvider/ProviderExtensions.cs	92.00%	0 Missing and 2 partials ⚠️
...er/Strategies/Models/StrategyPerProviderContext.cs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #488      +/-   ##
==========================================
+ Coverage   88.48%   89.67%   +1.18%     
==========================================
  Files          50       64      +14     
  Lines        2102     2509     +407     
  Branches      245      296      +51     
==========================================
+ Hits         1860     2250     +390     
- Misses        191      199       +8     
- Partials       51       60       +9     

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[sergheevxo]

Do you have any idea when this will be implemented in the NuGet package?

[askpt]

Do you have any idea when this will be implemented in the NuGet package?

@sergheevxo I am actively working on #338, and this feature is next on my list.