Sign a document timestamp signature field using a Timestamping
The signature must be an unsigned RFC 3161 timestamp signature.
If you need to create such a signature you can do so using the
You need to set one of either the TimestampServiceUrl
or the CustomSigner
properties before calling this function.
If you are using a CustomSigner the delegate method will be
passed the raw SHA256 digest of the document contents.
The timestamp signing operation is queued so you will typically
want to call Doc.Save or Commit after calling this function.