Use available indexes when in `CaseTransformFunction`

We are seeing various cases where auto generated SQL from visualization tools perform a lot of transforms/filtering using `CASE WHEN` from within aggregation functions. However, if the original predicate in the `where clause` had low selectivity, we end up scanning a lot of data to perform the `CaseTransform` evaluation. It would be good to see if we can utilize available indexes (when possible) for evaluating these as well.

cc: @atris @kishoreg