Clarify and codify access event in windows and groupings

[Licenser]

Describe the problem you are trying to solve

Inside a window group we can't guarantee that an event is present when we emit, this prevents access to some of the event data at this time.

Describe the solution you'd like

We should ensure that for windowed selects we limit access to parts of the event that are either in an aggregate function or part of the group by statement