Refactor entity classes for non-nullable properties

Removed nullable types and `init` properties, replacing them with standard getters and setters in various entity classes. Updated properties like `DocumentPath`, `SendingProfile`, and `SignatureHost` to be non-nullable and added required attributes.

Modified the project file to include `net462` as a target framework and added a reference for `System.ComponentModel.Annotations`. Removed the `IUnique<int>` interface from several classes to simplify uniqueness management.

These changes improve data integrity and usability across the entity classes.

EnvelopeGenerator.Domain/Entities/EnvelopeReceiver.cs

@@ -6,9 +6,9 @@ namespace EnvelopeGenerator.Domain.Entities
     public class EnvelopeReceiver : EnvelopeReceiverBase
     {
         [ForeignKey("EnvelopeId")]
-        public Envelope? Envelope { get; set; }
+        public Envelope Envelope { get; set; }
 
         [ForeignKey("ReceiverId")]
-        public Receiver? Receiver { get; set; }
+        public Receiver Receiver { get; set; }
     }
 }