Extend ActivitiesClient

In working on the LogbookButton I would have a few improvements to the ActivitiesClient to make it usable for this use case.

1. It should be possible to change the activity.
2. There should be a way to get a list of activities associated with the instance in a consistent format (for example as a list of Activity objects) regardless of what is passed in (int/str/NamedActivity/etc).
3. A few methods of Client are missing from ActivitiesClient. For consistency I would include these too.

The difficulty of point 2 is that, combined with #2 (closed), if the RBAC token is invalid then it is not possible to lookup the Activity objects. I wrote a quick wrapper class which works around this:

https://gitlab.cern.ch/tlevens/accwidgets-logbook/-/blob/master/accwidgets/logbook/_model.py

Not sure if there is a better way...