Add support for low power applications

[tfranssen]

I am just starting to use this reference design template. Its a great start for many projects, well done!

It would be nice to have a low power state supported. Right now I added PSM features and I have to stop golioth system using
golioth_system_client_stop()
otherwise PSM state will never initiated due to communication to the cloud.

If I change the main loop to this:

while (true) {
	golioth_system_client_start();
	k_sem_take(&connected, K_FOREVER);		
	app_work_sensor_read();
	golioth_system_client_stop();		
	k_sleep(K_SECONDS(get_loop_delay_s()));
}

I get some errors due to failed observations:

[00:04:12.452,392] <err> app_dfu: Error while receiving desired FW update: -110
[00:04:12.452,423] <err> golioth: Error on Settings observation: -110
[00:04:12.452,453] <err> golioth: Error on RPC observation: -110
[00:04:12.452,484] <err> app_state: Failed to receive 'desired' endpoint: -110

I switched the async LightDB stream function to sync function without the callback and everything works as expected except for the RPC functionality since the device is not online. I did not test the DFU functionality. Sleep current is 50 uA using a custom board.

It would be nice to have a setting that enables or disables PSM.