Refactor name retrieval to use GetFullName method

Updated the `CertificateModel.vb` and `EmailData.vb` files to replace direct access to the `FullName` property with the `GetFullName()` method for retrieving creator, sender, and receiver names. This change enhances flexibility and maintains consistency across the codebase.

Additionally, added a new static `GetFullName` method in the `EGUserExtensions.cs` file to centralize the formatting of user names by concatenating the `Prename` and `Name` properties.

EnvelopeGenerator.Domain/Entities/EGUserExtensions.cs

@@ -26,5 +26,7 @@ namespace EnvelopeGenerator.Domain.Entities
                 DeletedWho = user.DeletedWho
             };
         }
+
+        public static string GetFullName(this User user) => $"{user.Prename} {user.Name}";
     }
 }