[DF] Handle DataFusion CASE operations

[jdye64]

Is your feature request related to a problem? Please describe.
The current DataFusion implementation lacks the ability to properly handle CASE operations in a SQL string, they are ignored. The logic to handle a DataFusion CASE needs to the added to the Dask-SQL Python code base.

Describe the solution you'd like
Properly RexConvert a SQL string containing a CASE operation

Describe alternatives you've considered
None

Additional context
None

[jdye64]

Closed with #502