drivers: modem_cellular: added APP RDY signal handling

Some modems (e.g. Quectel BG95/EG25-G, several SIMCom parts) print
“APP RDY” when the application core is ready.  Parse this line and
proceed from AWAIT_POWER_ON immediately, while keeping the existing
startup timeout as a fall-back.

* add MODEM_CELLULAR_EVENT_MODEM_READY + chat match
* open UART and attach chat in AWAIT_POWER_ON
* jump to SET_BAUDRATE / RUN_INIT_SCRIPT on event

Signed-off-by: Van Petrosyan <van.petrosyan@sensirion.com>