Bluetooth: Host: Add common handling of VS IQ report

Bluetooth Controller has a vendor specific extensions that allows it
to send IQ report events with IQ samples that are 8 bits or 16 bits
signed integer. To use that functionality, there is added common
handler of vendor specific events.

Vendor specific events handling is prioritized to be done by user
provided event handler. If that is not available, then Host generic
implementation enters.

Added vendor specific events that are handled by common Host code
are BT_HCI_EVT_VS_LE_CONNECTIONLESS_IQ_REPORT and BT_HCI_EVT_VS_LE-
_CONNECTION_IQ_REPORT.

The only difference between regular IQ report events is size of
IQ samples, hence implementation of IQ report events is changed to
use the same user callback. To avoid differentiation of user callbacks
new member sample_type was added to bt_df_per_adv_sync_iq_samples-
_report. Also sample member is changed to be a union, to allow easy
access to IQ samples without type casting.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>