Add channel test providers for Email, Slack, Teams, and Webhook

- Implemented EmailChannelTestProvider to generate email preview payloads.
- Implemented SlackChannelTestProvider to create Slack message previews.
- Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews.
- Implemented WebhookChannelTestProvider to create webhook payloads.
- Added INotifyChannelTestProvider interface for channel-specific preview generation.
- Created ChannelTestPreviewContracts for request and response models.
- Developed NotifyChannelTestService to handle test send requests and generate previews.
- Added rate limit policies for test sends and delivery history.
- Implemented unit tests for service registration and binding.
- Updated project files to include necessary dependencies and configurations.

src/StellaOps.DependencyInjection/ServiceBindingAttribute.cs

@@ -0,0 +1,64 @@
+using System;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace StellaOps.DependencyInjection;
+
+/// <summary>
+/// Declares how a plug-in type should be registered with the host dependency injection container.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
+public sealed class ServiceBindingAttribute : Attribute
+{
+    /// <summary>
+    /// Creates a binding that registers the decorated type as itself with a singleton lifetime.
+    /// </summary>
+    public ServiceBindingAttribute()
+        : this(null, ServiceLifetime.Singleton)
+    {
+    }
+
+    /// <summary>
+    /// Creates a binding that registers the decorated type as itself with the specified lifetime.
+    /// </summary>
+    public ServiceBindingAttribute(ServiceLifetime lifetime)
+        : this(null, lifetime)
+    {
+    }
+
+    /// <summary>
+    /// Creates a binding that registers the decorated type as the specified service type with a singleton lifetime.
+    /// </summary>
+    public ServiceBindingAttribute(Type serviceType)
+        : this(serviceType, ServiceLifetime.Singleton)
+    {
+    }
+
+    /// <summary>
+    /// Creates a binding that registers the decorated type as the specified service type.
+    /// </summary>
+    public ServiceBindingAttribute(Type? serviceType, ServiceLifetime lifetime)
+    {
+        ServiceType = serviceType;
+        Lifetime = lifetime;
+    }
+
+    /// <summary>
+    /// The service contract that should resolve to the decorated implementation. When <c>null</c>, the implementation registers itself.
+    /// </summary>
+    public Type? ServiceType { get; }
+
+    /// <summary>
+    /// The lifetime that should be used when registering the decorated implementation.
+    /// </summary>
+    public ServiceLifetime Lifetime { get; }
+
+    /// <summary>
+    /// Indicates whether existing descriptors for the same service type should be removed before this binding is applied.
+    /// </summary>
+    public bool ReplaceExisting { get; init; }
+
+    /// <summary>
+    /// When true, the implementation is also registered as itself even if a service type is specified.
+    /// </summary>
+    public bool RegisterAsSelf { get; init; }
+}