device: add CONFIG_LLEXT_EXPORT_DEV_IDS_BY_HASH option

This new option allows to export devices using identifiers generated
from the hash of the devicetree node path, instead of the device's
ordinal number. Identifiers generated this way are stable across
rebuilds.

Add new test cases to test this new option.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>