Add final stage to the logic of physical plan planning

Instead of adding the final stage boolean to the physical plan execution, I think this logic should be part of the physical plan planning. i.e. when planning a multi-stage count, instead of planning a `count(final=true)` operator, you would just plan a `sum()` operator at the final stage.

_Originally posted by @asubiotto in https://github.com/polarsignals/frostdb/pull/202#discussion_r976278288_
      