Added SkyType gradient

com.unity.render-pipelines.high-definition/HDRP/Sky/GradientSky/GradientSky.cs

 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
-    [SkyUniqueID(150)]
+    [SkyUniqueID((int)SkyType.Gradient)]
     public class GradientSky : SkySettings
     {
         public ColorParameter top = new ColorParameter(Color.blue, true, false, true);

com.unity.render-pipelines.high-definition/HDRP/Sky/VisualEnvironment.cs

     public enum SkyType
     {
         HDRISky = 1,
-        ProceduralSky = 2
+        ProceduralSky = 2,
+        Gradient,
     }
 
     // Keep this class first in the file. Otherwise it seems that the script type is not registered properly.