Added the ability to retrieve a mouse button by it's MouseButton enum

Packages/com.unity.inputsystem/InputSystem/Devices/Mouse.cs

@@ -1,3 +1,4 @@
+using System.ComponentModel;
 using System.Runtime.InteropServices;
 using UnityEngine.InputSystem.Controls;
 using UnityEngine.InputSystem.Layouts;
@@ -223,6 +224,30 @@ public class Mouse : Pointer, IInputStateCallbackReceiver
         /// <value>Control representing the mouse click count.</value>
         public IntegerControl clickCount { get; protected set;  }
 
+        /// <summary>
+        /// Retrieve a mouse button by its <see cref="MouseButton"/> enumeration
+        /// constant.
+        /// </summary>
+        /// <param name="button">Button to retrieve.</param>
+        /// <exception cref="InvalidEnumArgumentException"><paramref name="button"/> is not a valid mouse
+        /// button value.</exception>
+        public ButtonControl this[MouseButton button]
+        {
+            get
+            {
+                switch (button)
+                {
+                    case MouseButton.Left: return leftButton;
+                    case MouseButton.Right: return rightButton;
+                    case MouseButton.Middle: return middleButton;
+                    case MouseButton.Forward: return forwardButton;
+                    case MouseButton.Back: return backButton;
+                    default:
+                        throw new InvalidEnumArgumentException(nameof(button), (int)button, typeof(MouseButton));
+                }
+            }
+        }
+
         /// <summary>
         /// The mouse that was added or updated last or null if there is no mouse
         /// connected to the system.