samples: psa: persistent_key: add the persistent_key sample

Add a sample to demonstrate use of persistent keys in the PSA Crypto API.

The implementation of the PSA ITS API that allows storage of persistent
keys is provided either by the just-introduced secure storage subsystem
or by TF-M.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>