Allow reading the CAN bus status from a CAN bus device

The CAN bus status has four main levels:

• Error active (fully functional)
• Error warning
• Error passive (no longer sending)
• Bus off (no longer on bus)

One way to get bus status changes is by using the status frames, but sometimes it's easier to just query the current bus status from the backend.

Adding this function will most likely require adding a virtual function QCanBusDevice::busStatus() and is therefore not binary compatible so would have to wait for Qt6.