Background activities scheduling was moved to its own middleware where the actual scheduling happens instead of in the invoking middleware. Removed ScheduleBackgroundActivities.cs handler and added ScheduleBackgroundActivitiesMiddleware.cs instead. As a part of this change, restructured BackgroundActivityInvokerMiddleware to BackgroundActivityCollectorMiddleware, and updated all relevant references.
Minor modifications to a few other files include changes in their package dependencies and activity kind settings.
