[Feature] Make it possible to stop observing

[laurensvalk]

The Technic Hub can sometimes lock up when advertising and scanning at the same time (https://github.com/pybricks/support/issues/1419)

Since there may not be a solution due to the Bluetooth chip limitations, maybe we should have a way to stop and restart observing, just as we have for broadcasting.

Then the user can choose which one to use in their script.