todo: Digital Signing

[dwarring]

Would be nice to have the ability to digitally sign documents in various ways. See Adobe technical documentation: https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSig/Acrobat_DigitalSignatures_in_PDF.pdf.

There are some online services that claim to be able to verify digital signatures and could be used for verification purposes.

PDF::Class has the PDF::Signature class. I think we'll also need an independent module.

Note that verification is a harder problem that signing. All (or at least most) types of signatures need to be supported.