samples: net: modem: add modem power callbacks
samples: net: modem: add modem power callbacks

Register modem power callbacks to show how it can be
used to provide e.g modem power on and off sequences.

Signed-off-by: Bartosz Bilas <c9544010575ed4e48ae813e3d4bf273cb2e959c4@hotmail.com>

zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr