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 <van.petrosyan@sensirion.com>