`datafusion_proto` crate should have exhaustive match statements for handling `Expr`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When adding new Expr variants, we should also be adding code to datafusion_proto to handle them but we are not forced into doing this today because the match statement handles the _ case with an error.

Describe the solution you'd like
Support all current Expr variants in and remove the _ case.

Describe alternatives you've considered
None

Additional context
None

[andygrove]

Currently datafusion.proto only handles expressions and not logical plans, but that will have to change if we want to support subquery expressions. Ballista already has support for plans so maybe we move some of that code to DataFusion? Another option would be to look at Substrait but that may be a significant amount of work.

fyi @alamb @carols10cents @Ted-Jiang @thinkharderdev since you have contributed to this file

[alamb]

I would think we should move the code for serializing logical plans into datafusion.proto as well

[thinkharderdev]

Yeah, agreed. I think it makes sense to move all serialization into datafusion-proto. It might be useful as well if we move forward with breaking Ballista out into it's own repo to help ensure that constructs added to DataFusion are always serializable.

[Ted-Jiang]

+1 , remove the _ case, will give error at compile time.