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.