avformat: implement SChannel SSP TLS protocol

This implementation does not support TLS listen sockets and loading
CA/Certs from files.

The Windows API does not support loading PEM certs, and would either
require a manual loader or instead be limited to loading Windows PFX
certificates

TLS listen sockets would have to be implemented quite separately, as many
of the APIs are different for server-mode (as opposed to client mode).