Dispatches to the specific visit method for this MIME entity.
This default implementation for MimeKit.Cryptography.MultipartSigned nodes calls MimeKit.MimeVisitor.VisitMultipartSigned. Override this method to call into a more specific method on a derived visitor class of the MimeKit.MimeVisitor class. However, it should still support unknown visitors by calling MimeKit.MimeVisitor.VisitMultipartSigned.