Load encryption code and its dependencies conditionally

[blikblum]

The encryption support soon landing in #820 increases pdfkit bundle size by 10%. Pdfkit is already quite big: 480Kb minified.

It's good to make the extra dependencies opt-in, so users that do not use this feature does not carries the extra code size.

This could be accomplished by using a plugin or middleware interface. Or even just as a configurable static property

[Rotzbua]

I don't need the encryption function in the browser. Pdfkit is only used for data export.
I would appreciate it if this dependency is not activated by default and saves some traffic.