Bluetooth: extending API for OOB pairing

This change extends the Bluetooth API to support pairing with the OOB
data that are used in the Authentication Stage of the pairing procedure.
The LE Secure Connections specific API for OOB pairing is also added.
The general OOB API should be able to accomodate the LE legacy pairing
later on.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>