feat: add pickFields filter to calendar endpoint

The program calendar endpoint returns a lot of data. Most of the time the client does not need everything steering has to offer either because it already has the necessary data or because it doesn’t need them.

The pickFields query parameter allows the client to only output a subset of the available fields.

One of the program calendar endpoints fails in the CI. It doesn’t fail locally which I find a little irritating. One change that might be related: the includeVirtual filter was set as include_virtual and I fixed that, because the filters are camelized by djangorestframework_camel_case. Again the test succeeds locally (both with include_virtual and includeVirtual).