Fix SUM/MEAN aggregation type support. (#12503)

This PR closes #8399. We cleaned up the logic by fixing SUM/MEAN aggregation type support, which also eliminated `TODO` comments in the target type definitions.

We kept the restriction that rolling min/max requires fixed width types because min/max aggregations do support non-fixed width in other aggregation implementations (groupby does a argmin-and-gather approach on strings, for instance).

This PR is collaborative work with @karthikeyann.

Authors:
  - Bradley Dice (https://github.com/bdice)
  - Karthikeyan (https://github.com/karthikeyann)

Approvers:
  - Mark Harris (https://github.com/harrism)
  - David Wendt (https://github.com/davidwendt)

URL: #12503