After #9240 datafusion protobuf will will silently ignore partition_by options;

The task is

1. add code to serialize / deserialize the CopyTo::partition_by field, added in Support Copy To Partitioned Files #9240 to the existing serialization code (similarly to how copy_to is done) https://github.com/apache/arrow-datafusion/blob/f1dcbf978210be6aa69b07892cf47321905aeece/datafusion/proto/src/logical_plan/mod.rs#L850

2. Update the test in datafusion/proto/tests/cases/roundtrip_logical_plan.rs to send a non empty partition column list

Originally posted by @devinjdangelo in #9240 (comment)