Add multipart machines system (#35969)

Content.Client/Machines/Components/MultipartMachineGhostComponent.cs

@@ -0,0 +1,14 @@
+namespace Content.Client.Machines.Components;
+
+/// <summary>
+/// Component attached to all multipart machine ghosts
+/// Intended for client side usage only, but used on prototypes.
+/// </summary>
+[RegisterComponent]
+public sealed partial class MultipartMachineGhostComponent : Component
+{
+    /// <summary>
+    /// Machine this particular ghost is linked to.
+    /// </summary>
+    public EntityUid? LinkedMachine = null;
+}