DSS-2653: Find signature field to sign by partial name

Created by: lurbain-qs

In other places of the library, "pdDocument.getSignatureFields()" is usually called to retrieve signature fields and then the partial name of the fields is used to compare with the specified field ID.

But in this particular place, the signature field to sign is retrieved using "acroForm.getField(targetFieldId)" which expects the fully qualified name of the field.

Changing this would allow minimal support for nested fields.