Refactored the OpenFeature framework to introduce `OpenFeatureProviderBuilder`, enhancing support for dependency injection and provider management.

- Changed namespaces to align with DI abstractions.
- Made `FeatureCodes` public for broader accessibility.
- Added `InternalsVisibleTo` for testing and project references.
- Introduced `OpenFeatureProviderBuilder` for managing providers and policies.
- Added extension methods for provider and policy registration.
- Refactored `OpenFeatureBuilder` to inherit from `OpenFeatureProviderBuilder`.
- Consolidated shared functionality in `OpenFeatureProviderOptions`.
- Updated `FeatureBuilderExtensions` and `InMemoryProviderOptions` to use the new abstraction.
- Updated tests to reflect new method signatures and hierarchy.
- Removed redundant methods and properties, improving code organization.

These changes improve maintainability, extensibility, and alignment with modern DI patterns.

Simplified the `AddProvider` API by removing the `TFeatureProvider` generic type parameter and directly using the `FeatureProvider` type. Updated the `TOptions` parameter to ensure it derives from `OpenFeatureProviderOptions`.

Added a `<ProjectReference>` to `OpenFeature.csproj` in `OpenFeature.DependencyInjection.Abstractions.csproj`. Updated `OpenFeatureOptions` to `OpenFeatureProviderOptions` in the default name selector policy.

Refactored `AddInMemoryProvider` methods to align with the new API. Updated tests in `OpenFeatureBuilderExtensionsTests` to reflect the changes and validate the updated functionality.

Performed general code cleanup, including XML documentation updates, to improve clarity and maintain consistency across the codebase.

Refactored `FeatureLifecycleManager` to modularize provider, hook, and handler initialization with new methods: `InitializeProvidersAsync`, `InitializeHooks`, and `InitializeHandlers`. Updated `EnsureInitializedAsync` to use these methods for improved readability and maintainability.

Revised `AddInMemoryProvider` in `FeatureBuilderExtensions` to use a generic `FeatureProvider` abstraction. Adjusted `CreateProvider` methods accordingly.

Improved code clarity in `FeatureFlagIntegrationTest` by renaming variables for consistency and removing redundant assignments.

Updated FeatureLifecycleManagerTests to replace OpenFeatureOptions
with OpenFeatureProviderOptions for configuring feature providers.
Added support for hooks and keyed singletons to enhance modularity.
Introduced additional feature flag retrieval in FeatureFlagIntegrationTest.
Added dependency on OpenFeature.DependencyInjection.Abstractions.

…hub.com/arttonoyan/dotnet-sdk into feature/di-abstractions-builder-api-587

Co-authored-by: André Silva <[email protected]>
Signed-off-by: Artyom Tonoyan <[email protected]>

Refactored `OpenFeatureOptions` to `OpenFeatureProviderOptions`
to improve clarity and maintainability. Updated all references
and test cases to use the new class. Removed inheritance of
`OpenFeatureOptions` from `OpenFeatureProviderOptions` and
introduced `_hookNames` for managing hook names.

Replaced `TestOptions` with `TestProviderOptions` in
`OpenFeatureBuilderExtensionsTests`, adding a `SomeFlag`
property for more flexible testing. Renamed and updated
`OpenFeatureOptionsTests` to `OpenFeatureProviderOptionsTests`.

Performed general cleanup, including removing unused
namespaces, ensuring consistent naming conventions, and
aligning method calls with the new class structure.

…hub.com/arttonoyan/dotnet-sdk into feature/di-abstractions-builder-api-587

Removed `OpenFeature.Hosting.Internal` namespace from
`FeatureLifecycleManagerTests.cs` and
`OpenFeatureBuilderExtensionsTests.cs` as it is no longer
required. Adjusted the placement of
`OpenFeature.Providers.DependencyInjection` for consistency
and improved readability. These changes streamline imports
and enhance maintainability.

Introduce cancellation checks in InitializeProvidersAsync to
allow prompt operation cancellation. Add null-coalescing
operator to ensure OpenFeatureProviderOptions are configured,
throwing an InvalidOperationException if not. Improve code
readability by adjusting formatting.