Represents the "Type" entry of the signature validation-related
information dictionary object.
It is an optional entry defined as part of the PDF 2.0
It contains a string representing a PDF name object.
If provided, this item must always take the value "VRI".
For definitive details see:.
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 262, page 584.