esp32/network_ppp: Add authentication support to the PPP interface.

This commit adds the connect() method to the PPP interface and requires
that connect() be called after active(1).  This is a breaking change for
the PPP API.

With the connect() method it's now possible to pass in authentication
information for PAP/CHAP, eg:

    ppp.active(1)
    ppp.connect(authmode=ppp.AUTH_PAP, username="user", "password="password")

If no authentication is needed simply call connect() without any
parameters.  This will get the original behaviour of calling active(1).