feat(dui3): adds layer concept in core as a poc

src/Speckle.Core/Models/Collection.cs

@@ -33,8 +33,9 @@ public class Collection : Base
   public string name { get; set; }
 
   /// <summary>
-  /// The type of this collection
+  /// The type of this collection. Note: Claire and Dim would propose we deprecate this prop. Do not use, please! 
   /// </summary>
+  [Obsolete("Note: Claire and Dim would propose we deprecate this prop. Do not use, please! Let's have a discussion about subclassing for your needs if nothing exists already.")]
   public string collectionType { get; set; }
 
   /// <summary>

src/Speckle.Core/Models/Layer.cs

@@ -0,0 +1,9 @@
+namespace Speckle.Core.Models;
+
+/// <summary>
+/// A specialized collection that represents a CAD-app layer. We expect this to grow in the future with possibly other shared props.
+/// </summary>
+public class Layer : Collection
+{
+  public int Color { get; set; }
+}