feat: anchor for non-ordinal reference to PlanRel

This adds an "anchor" to `PlanRel` that can be referenced from a
`ReferenceRel`. This anchor and reference relationship provides a
non-ordinal method for identifying and accessing a "subtree" or
sub-graph.

This commit leaves in the original `subtree_ordinal` attribute since it
seems a (mildly) more performant method for referencing a subtree, but
also since it is still relevant in the typical case. The new anchor
improves cases where multiple plans are merged and at least one
already contains a `ReferenceRel`.

It is expected that only one of `subtree_ordinal` or `subtree_reference`
will be used, however I don't see a good reason to enforce the use of
only one, so I did not group the attributes in a `oneof` constraint.

Issue: #725

proto/substrait/algebra.proto

@@ -1633,8 +1633,12 @@ message AggregateFunction {
   }
 }
 
-// This rel is used  to create references,
+// 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;
+
+  // points to a subtree_anchor in PlanRel (for non-ordinal reference)
+  uint32 subtree_reference = 2;
 }

proto/substrait/plan.proto

@@ -19,6 +19,9 @@ message PlanRel {
     // The root of a relation tree
     RelRoot root = 2;
   }
+
+  // provides a non-ordinal anchor for ReferenceRel
+  uint32 subtree_anchor = 3;
 }
 
 // Describe a set of operations to complete.