Add easy-to-use interfaces for GnuPG components (scdaemon, dirmngr)

Protocol reverse-engineering is not fun, but a lot of GnuPG internals are not documented. Why force users to go into this cryptic legacy hell when you are already doing this?

Maybe this library will eventually grow enough big to become a base for Pure Go GPGME :P

- [x] pinentry
Password prompts. Already implemented because this is reason why I created go-assuan: I wanted to implement a custom pinentry (see [ttyprompt](github.com/foxcpp/ttyprompt) repo).
- [ ] gpg-agent
Private key storage.
- [ ] dirmngr
Public key storage, keyserver interaction.
- [ ] scdaemon
Smartcards interaction (I don't have one so I don't think I will be able to correctly implement it).

If you have anything related to how these components work - I will greatly appreciate any hints.