support variant service match mode

[zhiyuanliang-ms]

Why this PR?

A follow up PR for #606, trying to resolve #604

Today VariantServiceProvider<T> only selects an implementation based on the assigned variant name. This PR adds an alternative mode where the implementation is selected based on the feature flag's enabled status (true/false), useful when a feature has no variants (e.g. a simple toggle flag).

Introduce VariantServiceMatchMode enum which has Variant (default) and Status value.

Usage:

// Service implementations
[VariantServiceAlias(true)]
public class EnabledCalculator : ICalculator { /* ... */ }

[VariantServiceAlias(false)]
public class DisabledCalculator : ICalculator { /* ... */ }

// Registration (keyed for lazy instantiation)
services.AddKeyedSingleton<ICalculator, EnabledCalculator>(true);
services.AddKeyedSingleton<ICalculator, DisabledCalculator>(false);

// Or normal registration
// services.AddSingleton<ICalculator, EnabledCalculator>();
// services.AddSingleton<ICalculator, DisabledCalculator>();

services.AddFeatureManagement()
    .WithVariantService<ICalculator>("AdvancedMath", VariantServiceMatchMode.Status);

Note that the AddKeyed* family takes object? serviceKey, not just string.

[Stepami]

what if i want to register my feature services against other keys?

[zhiyuanliang-ms]

I didn't test it. But my expectation is that you can register the same service multiple times using different keys,

[Stepami]

I'm trying to tell that if I want to use some key other than the feature status the variant sp won't get my impl as it doesn't know and can't know it.

The point is for VariantSp to be able to resolve my service not just with boolean key but any user-provided