drivers: modem_cellular: make APN configurable at run-time

* Implement the new cellular_set_apn() driver hook.
  The API validates the string, stores it in the driver’s
  private data and raises an internal event so the state
  machine can resume.

* Insert MODEM_CELLULAR_STATE_WAIT_FOR_APN between DLCI2 open
  and the APN-script step.
  – If CONFIG_MODEM_CELLULAR_APN is defined and non-empty the
    driver skips WAIT_FOR_APN (behaves exactly as before).
  – Otherwise the driver parks in WAIT_FOR_APN until the
    application calls cellular_set_apn().

* Add event-callback support (CELLULAR_EVENT_MODEM_INFO_CHANGED)
  so an app can subscribe to IMSI availability and kick its own
  APN selection logic.

Signed-off-by: Van Petrosyan <[email protected]>

Author: petrosyan-van

drivers/modem/Kconfig.cellular

@@ -40,8 +40,11 @@ config MODEM_CELLULAR
 if MODEM_CELLULAR
 
 config MODEM_CELLULAR_APN
-	string "APN"
+	string "Static APN"
 	default "internet"
+	help
+	  If left empty the driver will wait for the application to call
+	  cellular_set_apn() before it proceeds to the dial phase.
 
 config MODEM_CELLULAR_PERIODIC_SCRIPT_MS
 	int "Periodic script interval in milliseconds"