New: Add interface-filtered object references

Add native hard and soft reference types for scene objects that implement
a scripting interface:

- ScriptingObjectInterfaceReference<T>
- SoftObjectInterfaceReference<T>

Source/Engine/Scripting/Attributes/Editor/SoftObjectInterfaceReferenceAttribute.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Wojciech Figat. All rights reserved.
+
+using System;
+
+namespace FlaxEngine
+{
+    /// <summary>
+    /// Marks a generated interface property as a native soft object interface reference.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
+    public sealed class SoftObjectInterfaceReferenceAttribute : Attribute
+    {
+    }
+}