Add CAN bus utilization tracking

### 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.