Refactor DTOs for improved structure and documentation

Transitioned from records to classes for flexibility, added XML documentation for clarity, and updated property definitions to use standard getters and setters. Introduced the `required` keyword for essential properties, removed unnecessary constructors, and enhanced property descriptions for better readability. Additionally, overridden `GetHashCode` in `ReceiverReadDto` for proper collection behavior.

EnvelopeGenerator.Application/DTOs/Receiver/ReceiverCreateDto.cs

@@ -20,7 +20,7 @@ public record ReceiverCreateDto
     }
 
     [EmailAddress]
-    public string EmailAddress { get; init; }
+    public required string EmailAddress { get; init; }
 
     public string? TotpSecretkey { get; init; }