Add PPP over Serial (PPPoS) support

[amotl]

Dear Stefano,

this is our first attempt of providing the PPPoS implementation we have been discussing within [1] ff. and [2]. Please note this is still untested as I don't have respective hardware on my workbench.

With kind regards, Andreas.

[1] https://community.hiveeyes.org/t/unlocking-and-improving-the-pythings-sim800-gprs-module-for-micropython/2978/20 [2] https://community.hiveeyes.org/t/ppp-over-serial-pppos-support-for-micropython-on-esp32/2994

cc @ClemensGruber, @poesel, @robert-hh