Factor out the WiFiClient/Server objects

[devyte]

The WiFiClient/Server should be factored out. This can be done by allowing a constructor that receives the object already prepared beforehand. An example is the http client. Alternatively, you can use an approach like the webserver. The CAs or fingerprint or whatever can then be set up before passing in. As in other such cases, the FTP client/server would not own the obkect passed in and so shouldn't destroy it.