[Image Resizer] Upgraded to .NET Core 3.1 (#7317)

* project converted to sdk style

* image resizer core31

* image resizer test core31

* project and setup fixes

src/modules/imageresizer/ui/Helpers/Observable.cs

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+
+namespace ImageResizer.Helpers
+{
+    public class Observable : INotifyPropertyChanged
+    {
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        protected void Set<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
+        {
+            if (Equals(storage, value))
+            {
+                return;
+            }
+
+            storage = value;
+            OnPropertyChanged(propertyName);
+        }
+
+        protected void OnPropertyChanged(string propertyName) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+    }
+}