Add support for Digital Signatures

[hsyl20]

HPDF lacks support for Digital Signatures. It would be nice to add it.

Resources:

• https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSigDC/Acrobat_DigitalSignatures_in_PDF.pdf

[mgajda]

I understand there are three possible items:

1. Adding entries to PDF dictionary with signatures (this ticket)
2. Assuring that all content is embedded, to guarantee invariant rendering over time (possibly letting embedded fonts pass through).
3. If PDF is a PDF form, then adding visual clues about signature within relevant field.

Reference document from Adobe

#migamake