extract S/MIME functionality into a library? (end of 2022)

[tomholub]

I'm starting to feel like we are writing some sort of an S/MIME library here. It may be worth to one day extract this functionality, including their own tests, and make an actual library from it that uses forge as dependency and get it security-reviewed. Then we'd import this s/mime library into our project. Either that, or contribute the improvements back to forge. Because these S/MIME operations are getting quite dense: ideally the browser extension code would work at a higher level of abstraction.

For now, it's more convenient to keep it in this codebase, for flexibility as you develop and add major S/MIME functionality. But once that mostly settles, it would be better to extract it into a library.

Originally posted by @tomholub in #3979 (review)