Add GetKeyedOrDefault extension to IServiceProvider

[OmerMicro]

This PR adds the GetKeyedOrDefault<T> extension method to IServiceProvider.
This method attempts to retrieve a keyed service and falls back to GetRequiredService<T> if the keyed service is not found.
This is useful for scenarios where a default implementation should be used if a specific keyed implementation is not available.

Changes:

• Added GetKeyedOrDefault<T> to ServiceProviderKeyedServiceExtensions.cs
• Added unit tests in KeyedDependencyInjectionSpecificationTests.cs

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[huoyaoyuan]

Thank you for your interest of contribution. Adding new API like methods require going through API review process.

This method doesn't follow the contract of IServiceProvider that keyed and non-keyed services are separated.

[OmerMicro]

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