Add Animator property to Setter

src/Avalonia.Animation/Animation.cs

@@ -248,7 +248,7 @@ namespace Avalonia.Animation
             {
                 foreach (var setter in keyframe.Setters)
                 {
-                    var handler = GetAnimatorType(setter.Property);
+                    var handler = setter.Animator ?? GetAnimatorType(setter.Property);
 
                     if (handler == null)
                     {

src/Avalonia.Animation/IAnimationSetter.cs

@@ -1,8 +1,11 @@
+using System;
+
 namespace Avalonia.Animation
 {
     public interface IAnimationSetter
     {
         AvaloniaProperty Property { get; set; }
         object Value { get; set; }
+        Type Animator { get; set; }
     }
 }

src/Avalonia.Styling/Styling/Setter.cs

@@ -59,6 +59,11 @@ namespace Avalonia.Styling
             }
         }
 
+        /// <summary>
+        /// Gets or sets the property animator.
+        /// </summary>
+        public Type? Animator { get; set; }
+
         public ISetterInstance Instance(IStyleable target)
         {
             target = target ?? throw new ArgumentNullException(nameof(target));