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

This is a synchronous software API which wraps the existing software
implementation in libsodium. This is different from the code in main
in that this uses libsodium directly. The version in main uses the
software backend shared with OCF, but main required changes that break
the ABI of struct enc_xform that cannot be merged to stable/13.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit e71680049bb8ff395aeaa144377dd9e49331f45e)