Support predicate pruning on `Expr::Case` expressions

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
In certain situations , IOx is likely going to make predicates that look like the following

CASE 
  WHEN col IS NULL THEN '' 
  ELSE col 
END

that basically map null to the empty string

We would like to use such predicates in order to prune out Chunks (or parquet record groups)

Describe the solution you'd like
I would like CaseExpr to be added to the list of expression types supported by predicate pruning in build_predicate_expression:
https://github.com/apache/arrow-datafusion/blob/03075d5f4b3fdfd8f82144fcd409418832a4bf69/datafusion/src/physical_optimizer/pruning.rs#L640-L699

The tricky bit of this PR would be figuring out what the transformation is and in what circumstances it can be applied

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
See https://github.com/influxdata/influxdb_iox/pull/3557 for more details

[tustvold]

@alamb could you assign this ticket to me please 😄