Add XR sample assets and update settings

Assets/Samples/XR Plugin Management/4.5.3/Example XR Management implementation/SampleConstants.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace Samples
+{
+    /// <summary>
+    /// Static constants
+    /// </summary>
+    public static class SampleConstants
+    {
+        /// <summary>
+        /// Key we use to store and retrieve custom configuration settings from EditorBuildSettings
+        /// </summary>
+        public const string k_SettingsKey = "com.unity.xr.management.sample_settings";
+    }
+}