snippets: Add nus-console snippet

Allowing to be applied on various samples to enable UART over Bluetooth
LE NUS without added complexity.
Tested with nrf52840dk/nrf52840 on the following samples:
- samples/subsys/console/echo.
- samples/subsys/console/getchar.
- samples/subsys/console/getline.
- samples/subsys/logging/logger.

Signed-off-by: Luis Ubieda <luisf@croxel.com>