SSLContext builtin to the WifiClientSecure.cpp file makes it harder to extend WifiClientSecure class

[schlaegerz]

I wanted to make some very specific changes to the WifiClientSecure class, but since SSLContext is defined completely inside the WifiClientSecure.cpp file it makes it significantly harder to deal with it.

It would be a great help to define the SSLContext class in a header so I don't have to reimplement everything if I want to extend the class.