ssh-key: support for reading PPK format private keys

[Eugeny]

This PR implements full support for PuTTY's version 2 and 3 key formats, including encryption and all supported key types.

Format documentation: https://tartarus.org/~simon/putty-snapshots/htmldoc/AppendixC.html#ppk

New code and deps are gated behind the ppk feature:

• argon2 - decryption KDF
• hmac - key MAC
• hex - for parsing the embedded MAC

Keys are decrypted immediately on parse, as PPK can't be cleanly integrated into the KeyData::Encrypted architecture because the existing decryption code just assumes OpenSSH format. PPK's private key part does not include the key elements present in the public key and has both different order and format of the fields (except for DSA)

New code is mostly confined in a separate ppk.rs mod

Tests included