Add support for EnableStopPropagation and EnablePreventDefault.

- For our current world Blazor both of these properties are always `true`. However, for other non-web Blazor scenarios the `EventHandler` attributes at that layer will turn off these features because they're specific to web.

#14517

Author: N. Taylor Mullen

src/Components/Components/ref/Microsoft.AspNetCore.Components.netcoreapp.cs

@@ -207,7 +207,10 @@ public readonly partial struct EventCallback<TValue>
     public sealed partial class EventHandlerAttribute : System.Attribute
     {
         public EventHandlerAttribute(string attributeName, System.Type eventArgsType) { }
+        public EventHandlerAttribute(string attributeName, System.Type eventArgsType, bool enableStopPropagation, bool enablePreventDefault) { }
         public string AttributeName { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public bool EnablePreventDefault { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public bool EnableStopPropagation { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public System.Type EventArgsType { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
     }
     public partial interface IComponent

src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs

@@ -207,7 +207,10 @@ public readonly partial struct EventCallback<TValue>
     public sealed partial class EventHandlerAttribute : System.Attribute
     {
         public EventHandlerAttribute(string attributeName, System.Type eventArgsType) { }
+        public EventHandlerAttribute(string attributeName, System.Type eventArgsType, bool enableStopPropagation, bool enablePreventDefault) { }
         public string AttributeName { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public bool EnablePreventDefault { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public bool EnableStopPropagation { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public System.Type EventArgsType { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
     }
     public partial interface IComponent

src/Components/Components/src/EventHandlerAttribute.cs

@@ -16,7 +16,18 @@ public sealed class EventHandlerAttribute : Attribute
         /// </summary>
         /// <param name="attributeName"></param>
         /// <param name="eventArgsType"></param>
-        public EventHandlerAttribute(string attributeName, Type eventArgsType)
+        public EventHandlerAttribute(string attributeName, Type eventArgsType) : this(attributeName, eventArgsType, false, false)
+        {
+        }
+
+        /// <summary>
+        /// Constructs an instance of <see cref="EventHandlerAttribute"/>.
+        /// </summary>
+        /// <param name="attributeName"></param>
+        /// <param name="eventArgsType"></param>
+        /// <param name="enableStopPropagation"></param>
+        /// <param name="enablePreventDefault"></param>
+        public EventHandlerAttribute(string attributeName, Type eventArgsType, bool enableStopPropagation, bool enablePreventDefault)
         {
             if (attributeName == null)
             {
@@ -30,6 +41,8 @@ public EventHandlerAttribute(string attributeName, Type eventArgsType)
 
             AttributeName = attributeName;
             EventArgsType = eventArgsType;
+            EnableStopPropagation = enableStopPropagation;
+            EnablePreventDefault = enablePreventDefault;
         }
 
         /// <summary>
@@ -41,5 +54,15 @@ public EventHandlerAttribute(string attributeName, Type eventArgsType)
         /// Gets the event argument type.
         /// </summary>
         public Type EventArgsType { get; }
+
+        /// <summary>
+        /// Gets the event's ability to stop propagation.
+        /// </summary>
+        public bool EnableStopPropagation { get; }
+
+        /// <summary>
+        /// Gets the event's ability to prevent default event flow.
+        /// </summary>
+        public bool EnablePreventDefault { get; }
     }
 }