Type forward system events (Azure#50371)

* WIP

* WIP

* clean up

* wip

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* wip

* regen

* wip

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* WIP

* regen

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* Rev major version

* api file

* update commit

Author: JoshLove-msft

sdk/eventgrid/Azure.Messaging.EventGrid.SystemEvents/api/Azure.Messaging.EventGrid.SystemEvents.net8.0.cs

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+namespace Azure.Messaging.EventGrid.SystemEvents
+{
+    internal static class Constants
+    {
+        internal static class MediaEvents
+        {
+            internal const string NotApplicable = "n/a";
+        }
+    }
+}

sdk/eventgrid/Azure.Messaging.EventGrid.SystemEvents/api/Azure.Messaging.EventGrid.SystemEvents.netstandard2.0.cs

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+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+
+namespace Azure.Messaging.EventGrid.SystemEvents
+{
+    public partial class AcsChatThreadCreatedEventData
+    {
+        /// <summary>
+        /// Gets the properties of the chat thread created event.
+        /// </summary>
+        public IReadOnlyDictionary<string, object> Properties { get; }
+    }
+}

sdk/eventgrid/Azure.Messaging.EventGrid.SystemEvents/src/Constants.cs

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+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+
+namespace Azure.Messaging.EventGrid.SystemEvents
+{
+    public partial class AcsChatThreadCreatedWithUserEventData
+    {
+        /// <summary>
+        /// Gets the properties of the chat thread created with user event.
+        /// </summary>
+        public IReadOnlyDictionary<string, object> Properties { get; }
+    }
+}

sdk/eventgrid/Azure.Messaging.EventGrid.SystemEvents/src/Customization/AcsChatThreadCreatedEventData.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+
+namespace Azure.Messaging.EventGrid.SystemEvents
+{
+    public partial class AcsChatThreadPropertiesUpdatedEventData
+    {
+        /// <summary>
+        /// Gets the properties of the chat thread updated event.
+        /// </summary>
+        public IReadOnlyDictionary<string, object> Properties { get; }
+    }
+}

sdk/eventgrid/Azure.Messaging.EventGrid.SystemEvents/src/Customization/AcsChatThreadCreatedWithUserEventData.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+
+namespace Azure.Messaging.EventGrid.SystemEvents
+{
+    public partial class AcsChatThreadPropertiesUpdatedPerUserEventData
+    {
+        /// <summary>
+        /// The properties of the chat thread that were updated for the user.
+        /// </summary>
+        public IReadOnlyDictionary<string, object> Properties { get; }
+    }
+}