esp_modem: Fixed race condition on exiting PPP mode

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

Closes https://github.com/espressif/esp-idf/issues/6013
Closes https://github.com/espressif/esp-idf/issues/5737
Closes https://github.com/espressif/esp-idf/issues/6024
Closes https://github.com/espressif/esp-idf/issues/6058
Closes https://github.com/espressif/esp-idf/issues/5563
Closes https://github.com/espressif/esp-idf/issues/5695
Closes https://github.com/espressif/esp-idf/issues/5633
Closes https://github.com/espressif/esp-idf/issues/4482
Related https://github.com/espressif/esp-idf/pull/4849
Related https://github.com/espressif/esp-idf/pull/4653

examples/protocols/pppos_client/components/modem/include/esp_modem.h

@@ -158,6 +158,17 @@ esp_err_t esp_modem_stop_ppp(modem_dte_t *dte);
  */
 esp_err_t esp_modem_set_rx_cb(modem_dte_t *dte, esp_modem_on_receive receive_cb, void *receive_cb_ctx);
 
+/**
+ * @brief Notify the modem, that ppp netif has closed
+ *
+ * @note This API should only be used internally by the modem-netif layer
+ *
+ * @param dte ESP Modem DTE object
+ *
+ * @return ESP_OK on success
+ */
+esp_err_t esp_modem_notify_ppp_netif_closed(modem_dte_t *dte);
+
 #ifdef __cplusplus
 }
 #endif

examples/protocols/pppos_client/components/modem/include/esp_modem_dte.h

@@ -30,7 +30,9 @@ typedef struct modem_dce modem_dce_t;
  */
 typedef enum {
     MODEM_COMMAND_MODE = 0, /*!< Command Mode */
-    MODEM_PPP_MODE          /*!< PPP Mode */
+    MODEM_PPP_MODE,         /*!< PPP Mode */
+    MODEM_TRANSITION_MODE   /*!< Transition Mode between data and command mode indicating that
+                                 the modem is not yet ready for sending commands nor data */
 } modem_mode_t;
 
 /**