[SFTP] Support for SHA-2 Kex and HMAC

SFTP plugin currently supports only SHA-1 variations of key exchange algorithms and HMACs - support for SHA-2 variations (like `diffie-hellman-group-exchange-sha256`, `curve25519-sha256` for Kex and `hmac-sha2-512`, `hmac-sha2-256`, etc. for HMAC) would be great security improvement.
