Add support for Argon2 S2K

[lubux]

Adds support for encrypting and decrypting messages or keys with Argon2 S2K as specified in draft-ietf-openpgp-crypto-refresh.

• Added Argon2 in the s2k package and refactored it.
• Introduces a new field S2KConfig in packet.Config that allows configuring S2K. All configurations for S2K should now be done via an S2K configuration linked in this field. The config remains backward compatible to setting S2KCount directly in packet.Config.
• The public Encrypt method exposed in packet.PrivateKey for encrypting keys currently does not allow configuring S2K and defaults to the Iterative mode. There is now a private function that allows configuration. (Should be adapted in the future)

[lubux]

Thanks for the inputs. I addressed the comments.