samples: Add sample 'application_development/out_of_tree_driver'

Add a sample that demonstrates (and tests) that custom drivers can be
maintained outside of Zephyr.

The sample is fairly minimal with few dependencies and should
therefore be very portable. It also includes a sample.yaml that should
ensure that it does not regress.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>