feedback-task: created ref_type oneof field

Created a oneof field, "ref_type," (should expand to `ref_type_case` for
generated code) that captures both types of references. The old type,
subtree_ordinal, has also been deprecated.

This addresses feedback in the PR tagged below.

PR: #726

proto/substrait/algebra.proto

@@ -1636,9 +1636,11 @@ message AggregateFunction {
 // This rel is used to create references,
 // in case we refer to a RelRoot field names will be ignored
 message ReferenceRel {
-  // points to a PlanRel in Plan.relations (ordinal reference)
-  int32 subtree_ordinal = 1;
+  oneof ref_type {
+    // points to a PlanRel in Plan.relations (ordinal reference)
+    int32 subtree_ordinal = 1 [deprecated = true];
 
-  // points to a subtree_anchor in PlanRel (for non-ordinal reference)
-  uint32 subtree_reference = 2;
+    // points to a subtree_anchor in PlanRel (for non-ordinal reference)
+    uint32 subtree_reference = 2;
+  }
 }