manifest: Get BabbleSim via the manifest

To facilitate users experience, fetch the required
BabbleSim components using the west manifest.
Until now, users would need to either use the Zephyr
docker image, or fetch bsim on their own.
For many years there has been a request to automate this
process. This is the first step.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>