Updated the `using` directives in `SignatureDto.cs` to include the `EnvelopeGenerator.Domain.Constants` namespace and reordered the existing namespaces. Added a new `SenderAppType` property to the `SignatureDto` class, with a default value of `SenderAppType.LegacyFormApp`.
A new property `Annotations` of type `IEnumerable<AnnotationDto>?`
was added to the `SignatureDto` class. This property includes a
getter and setter, allowing the class to manage a collection of
annotations. The property is nullable, enabling it to hold a
`null` value if no annotations are present.
