Implement support for ASP.NET core Feature Management

[haacked]

This includes support for variants as well.

Fixes #37

Feature Management

PostHog.AspNetCore supports .NET Feature Management. This allows you to use the <feature /> tag helper and
the FeatureGateAttribute in your ASP.NET Core applications to gate access to certain features using PostHog
feature flags.

Setup

To use feature flags with the .NET Feature Management library, you'll need to implement the
IPostHogFeatureFlagContextProvider interface. The quickest way to do that is to inherit from the
PostHogFeatureFlagContextProvider class and override the GetDistinctId and GetFeatureFlagOptionsAsync methods.

public class MyFeatureFlagContextProvider(IHttpContextAccessor httpContextAccessor)
    : PostHogFeatureFlagContextProvider
{
    protected override string? GetDistinctId() => httpContextAccessor.HttpContext?.User.Identity?.Name;
    
    protected override ValueTask<FeatureFlagOptions> GetFeatureFlagOptionsAsync()
    {
        // In a real app, you might get this information from a database or other source for the current user.
        return ValueTask.FromResult(
            new FeatureFlagOptions
            {
                PersonProperties = new Dictionary<string, object?>
                {
                    ["email"] = "[email protected]"
                },
                OnlyEvaluateLocally = true
            });
    }
}

Then, register your implementation in Program.cs (or Startup.cs):

var builder = WebApplication.CreateBuilder(args);
builder.AddPostHog(options => {
    options.UseFeatureManagement<MyFeatureFlagContextProvider>();
});

Usage

You can now use feature tag helpers in your Razor views:

<feature name="awesome-new-feature">
    <p>This is the new feature!</p>
</feature>
<feature name="awesome-new-feature" negate="true">
    <p>Sorry, no awesome new feature for you.</p>
</feature>

Multivariate feature flags are also supported:

<feature name="awesome-new-feature" value="variant-a">
    <p>This is the new feature variant A!</p>
</feature>
<feature name="awesome-new-feature" value="variant-b">
    <p>This is the new feature variant B!</p>
</feature>

You can also use the FeatureGateAttribute to gate access to controllers or actions:

[FeatureGate("awesome-new-feature")]
public class NewFeatureController : Controller
{
    public IActionResult Index()
    {
        return View();
    }
}

[greptile-apps]

PR Summary

This PR implements ASP.NET Core Feature Management support in the PostHog .NET SDK, enabling feature flag management through tag helpers and attributes in ASP.NET Core applications.

• Added PostHogFeatureFlagContextProvider base class in /src/PostHog.AspNetCore/FeatureManagement/ for evaluating feature flags with customizable user identification and options
• Implemented PostHogVariantFeatureManager in /src/PostHog.AspNetCore/FeatureManagement/PostHogVariantFeatureManager.cs to handle multivariate feature flags and variants
• Created PostHogFeatureFilter and PostHogFeatureDefinitionProvider in /src/PostHog.AspNetCore/FeatureManagement/ to integrate with ASP.NET Core's feature management system
• Added comprehensive unit tests in /tests/UnitTests.AspNetCore/FeatureManagement/ covering feature flag evaluation, variants, and targeting
• Updated /src/PostHog.AspNetCore/README.md with detailed documentation on setup and usage of feature management capabilities

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