feat(Signature): add Annotations navigation property to Signature entity

- Added `IEnumerable<Annotation>? Annotations` to Signature class
- Added `using System.Collections.Generic;` for .NET Framework builds

EnvelopeGenerator.Domain/Entities/Annotation.cs

@@ -48,7 +48,11 @@ public class Annotation
         ChangedWho { get; set; }
 
     [ForeignKey("ElementId")]
-    public virtual Signature Element { get; set; }
+    public virtual Signature   
+#if NET
+        ?
+#endif     
+        Element { get; set; }
 }
 
 #if NETFRAMEWORK