Bluetooth: Add dynamic le scan listener interface

Adds a way to register listeners for incoming scanner packets, in
addition to the callback passed in bt_le_scan_enable.

This allows application modules to add multiple scan packet listeners
without owning the scanner life cycle API, enabling use cases like
beacon scanning alongside Bluetooth Mesh.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>