crypto: Add support for the XChaCha20-Poly1305 AEAD cipher.

This cipher is a wrapper around the ChaCha20-Poly1305 AEAD cipher
which accepts a larger nonce. Part of the nonce is used along with
the key as an input to HChaCha20 to generate a derived key used for
ChaCha20-Poly1305.

This cipher is used by WireGuard.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33523