Add logical plan support to `datafusion-proto`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
datafusion-proto has a protobuf definition of logical expressions and associated serde code to translate between the Expr enum and protobuf. I would like it to also support serializing logical plans.

Describe the solution you'd like
Ballista already contains a separate protobuf definition and serde code for logical plans that builds on top of a copy of the datafusion proto file so we should be able to move that to datafusion and then have Ballista use the datafusion-proto API for serializing plans.

Describe alternatives you've considered
None

Additional context
Add any other context or screenshots about the feature request here.