Add Chacha20-Poly1305 as a KTLS cipher suite.

Chacha20-Poly1305 for TLS is an AEAD cipher suite for both TLS 1.2 and
TLS 1.3 (RFCs 7905 and 8446). For both versions, Chacha20 uses the
server and client IVs as implicit nonces xored with the record
sequence number to generate the per-record nonce matching the
construction used with AES-GCM for TLS 1.3.

Reviewed by: gallatin
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D27839