class PDFOptions: new properties to sign existing signature fields

[SibrenJacobs]

• lock_form_except_signaturefield: locks/flattens the forms in the pdf except signature fields. Optional.
• output_sign_certificate_field: name of already existing certificate field that you want to sign. Optional.
• output_sign_certificate_background_image: certificate background image as a base64 string, URL, FTP location or a server path. Optional.