Bluetooth: Add support for vendor-specific events

Add an event handler for HCI vendor-specific events with the
event code BT_HCI_EVT_VENDOR.

A vendor defined callback can be registered to be called when
vendor-Specific events are received in the stack. The callback can then
decode and handle the event; if not the stack will decode and handle
the event.

Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>