Support chacha20-poly1305@openssh.com cipher

> **5586502** created the issue

This is in the line of tickets #8488, #8528 and #8537, but not as urgent as the key exchange algorithms. 

Apparently the ciphers aes256-ctr, aes192-ctr and aes128-ctr which are left after #8537 are still secure if Encrypt-then-MAC is used, but they do not offer authenticated encryption. Chacha20-poly1305 fixes this. 