Add placeholder SceneHandle object

Author: EmandM

com.unity.netcode.gameobjects/Runtime/SceneManagement/SceneHandle.cs

@@ -0,0 +1,59 @@
+#if !SCENE_MANAGEMENT_SCENE_HANDLE_AVAILABLE
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Unity.Netcode
+{
+    internal struct SceneHandle : IEquatable<SceneHandle>
+    {
+        private int m_Handle;
+        public static SceneHandle None => default;
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public bool Equals(SceneHandle other)
+        {
+            return m_Handle == other.m_Handle;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public override bool Equals(object obj)
+        {
+            return obj is SceneHandle other && Equals(other);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public override int GetHashCode()
+        {
+            return m_Handle;
+        }
+
+        /// <summary>
+        /// Test for equality.
+        /// </summary>
+        /// <param name="left"></param>
+        /// <param name="right"></param>
+        /// <returns>True if the two SceneHandles are the same</returns>
+        public static bool operator ==(SceneHandle left, SceneHandle right) => left.Equals(right);
+
+        /// <summary>
+        /// Test for inequality.
+        /// </summary>
+        /// <param name="left"></param>
+        /// <param name="right"></param>
+        /// <returns>True if the two SceneHandles are different</returns>
+        public static bool operator !=(SceneHandle left, SceneHandle right) => !left.Equals(right);
+
+        /// <summary>
+        /// Implicit conversion from <see langword="int"/> to <see cref="SceneHandle"/>.
+        /// </summary>
+        /// <param name="handle"></param>
+        public static implicit operator SceneHandle(int handle) => new() { m_Handle = handle };
+
+        /// <summary>
+        /// Implicit conversion from <see cref="SceneHandle"/> to <see langword="int"/>.
+        /// </summary>
+        /// <param name="handle">The SceneHandle</param>
+        public static implicit operator int(SceneHandle handle) => handle.m_Handle;
+    }
+}
+#endif