esp_modem_stop_ppp() stops both ppp netif and switches the modem back to
command mode. IF these two actions are not synchronised, we might
experience issues of
* active PPP session trying to send/receive uart-data
* command mode already active before modem switched to it
both resulting in crashes.
Fixed by introducing the transition mode and running these actions in sequence
* set esp-modem to transition mode
* enter command mode, wait for the reply or re-sync
* close the PPP netif
* wait until the netif closes
Other fixes include ignoring certain events if modem component not ready
or not in appropriate mode:
* ignoring all UART events comming from DTE with no DCE attached
* ignore pattern detection in PPP mode
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
