Add EnvShapeType.Sky

Assets/ScriptableRenderLoop/HDRenderLoop/Lighting/LightDefinition.cs

     [GenerateHLSL]
     public enum EnvShapeType
     {
-        None, 
-        Box, 
-        Sphere
+        None,
+        Box,
+        Sphere,
+        Sky
     };
 
     [GenerateHLSL]

Assets/ScriptableRenderLoop/HDRenderLoop/Lighting/LightDefinition.cs.hlsl

 #define ENVSHAPETYPE_NONE (0)
 #define ENVSHAPETYPE_BOX (1)
 #define ENVSHAPETYPE_SPHERE (2)
+#define ENVSHAPETYPE_SKY (3)
 
 //
 // UnityEngine.Experimental.ScriptableRenderLoop.EnvConstants:  static fields

Assets/ScriptableRenderLoop/HDRenderLoop/Material/Lit/Lit.hlsl

         float distFade = max(length(positionLS) - lightData.innerDistance.x, 0.0);
         weight.y = saturate(1.0 - distFade / max(lightData.blendDistance, 0.0001)); // avoid divide by zero
     }
-    else // ENVSHAPETYPE_BOX or ENVSHAPETYPE_NONE 
+    else if (lightData.envShapeType == ENVSHAPETYPE_BOX ||
+             lightData.envShapeType == ENVSHAPETYPE_NONE)
     {
         // Calculate falloff value, so reflections on the edges of the volume would gradually blend to previous reflection.
         float distFade = DistancePointBox(positionLS, -lightData.innerDistance, lightData.innerDistance);