drivers/sensor: sht3xd: implement single shot mode

For now there is only periodic data acquisition mode implemented.
This mode is quite power consuming. Based on datasheet in idle
state in periodic data acquisition mode SHT3X consumes 45uA but
in single shot mode 0.2uA. For many applications where power
consumption has to be kept as low as possible single shot mode
is the only choice. Tester on custom board NRF52832 + SHT31-DIS.

Signed-off-by: Michał Oleszczyk <oleszczyk.m@gmail.com>