Describe the bug

The FieldReference in Substrait is defined with root_type being oneof

  message FieldReference {

    // Whether this reference has an origin of a root struct or is based on the
    // ouput of an expression. When this is a RootReference and direct_reference
    // above is used, the direct_reference must be of a type StructField.
    oneof root_type {
      Expression expression = 3;
      RootReference root_reference = 4;
      OuterReference outer_reference = 5;
    }

A RootReference message is defined, that is empty but is used in most cases I've seen.

The code [here] (

When this is serialized to a protobuf and read by other engines (Substrait=Java in my case) this fails. with a
Unhandled type: ROOTTYPE_NOT_SET

To Reproduce

Create a DataFusion plan, and then try and consume it within say SubstraitJava

Expected behavior

The protobuf should be valid and parse correctly

Additional context

No response