create secure context to use cert client auth and validate server ca

[ymahlaoui]

This mod is for using cert client auth to syslog server and also validate the server cet

[DABH]

I think this would break things for users using secureProtocol, which doesn't seem ideal. Do you have any thoughts on that? Should we support both secureProtocol and secureContext fields? In any case, if we wanted to accept this PR, it would be very helpful to add a test and ideally some documentation, too, so people know what this is...