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

EnvelopeGenerator.Domain/Entities/Signature.cs

@@ -4,6 +4,7 @@ using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 #if NETFRAMEWORK
 using System;
+using System.Collections.Generic;
 #endif
 
 namespace EnvelopeGenerator.Domain.Entities
@@ -109,6 +110,12 @@ public class Signature : ISignature
     [ForeignKey("ReceiverId")]
     public virtual Receiver Receiver { get; set; }
 
+    public virtual IEnumerable<Annotation>
+#if NET
+        ?
+#endif
+        Annotations { get; set; }
+
 #if NETFRAMEWORK
     [NotMapped]
     public double Top => Math.Round(Y, 5);