Reuses most of the code from BG96. At this moment, this is not a
perfect support for other modules as the generic handlers for most
common modem functionality is not easily reusable. Refactoring to a
fully independent modem component will solve this.
Closes https://github.com/espressif/esp-idf/issues/5250
1. add support for ESP32 DTE
2. add support for SIM800/BG96 DCE
3. add PPPoS setup procedure
4. add support for SMS
5. add mqtt example after PPP connection established