Add CAN bus utilization tracking

[ilanbiala]

Is your feature request related to a problem? Please describe.

For some testing, it is helpful to know the current CAN bus load as tests are being performed, or for load testing.

Describe the solution you'd like

Having python-can be able to provide the utilization for the past N seconds (provided either at bus initialization and running on an ongoing basis, or something like a bus.start_monitoring_utilization(window_time_s=5.0) and then having a valid value for that 5s window) would be useful, rather than having to implement this above the bus level.

Describe alternatives you've considered

Implementing this above the bus level would lead to many duplicate implementations across projects.

Additional context

canbusload provides this feature, but not in a programmatic library call available in Python as far as I could tell.