refactor/sensor-pointers

[jacek-kow]

1. Sensor objects are now created as unique_ptrs that are passed into the Application constructor right from the start - instead of first creating static objects and then moving them inside the unique_ptrs. Code in main is thus simpler and easier to understand.
2. Zephyr device struct pointers obtained from DEVICE_DT_GET_ANY are now no longer parameters of the sensors' constructor. Instead, DEVICE_DT_GET_ANY is called inside the sensor class' constructor. This, again, makes main's code less cluttered.
   Note that this wasn't refactored in the Analog sensor, as the Analog sensor requires different specifications structs (obtained from ADC_DT_SPEC_GET) for different ADC sensors - that's why the soil sensor's specification wasn't hard-coded in the Analog constructor.
3. Internal device pointers in each Sensor class were kept as raw pointers, because DEVICE_DT_GET_ANY returns a device address from the device tree - no memory allocation is performed, so these pointers don't need to be deleted. Device pointers are not deleted in Zephyr samples, e. g. the BME280 sample.