Add an implementation of CHACHA20_POLY1305 to cryptosoft.

This uses the chacha20 IETF and poly1305 implementations from
libsodium. A seperate auth_hash is created for the auth side whose
Setkey method derives the poly1305 key from the AEAD key and nonce as
described in RFC 8439.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D27837