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
Basic set of configuration related to UART moved from KConfig
to runtime configuration structure to simplify reusing the component for
testing and examples.
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