crypto: Add a simple API for [X]ChaCha20-Poly1035 on flat buffers.

This is a synchronous software API which wraps the existing software
implementation shared with OCF. Note that this will not currently
use optimized backends (such as ossl(4)) but may be appropriate for
operations on small buffers.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33524