Change the type of `DurableTask.Core.History.HistoryEvent` so it is not accessed from the global namespace

Currently, the `DurableTask.Core.History.HistoryEvent` is accessed from the global namespace due to a naming conflict. Ideally, a shorter type name would be used, and there would be a cleaner way to access it than through the global namespace.

The issue names only DurableTask.Core.History.HistoryEvent and provides no files, tests, or entry points. First trace where this type is exposed and accessed, then clarify the intended shorter name and namespace behavior with maintainers; done requires an agreed API change that removes the global-namespace workaround.

Written by the indexing model from the issue text.