[WIP] feat: change grouping expressions in AggregateRel to references.

BREAKING CHANGE: This PR changes the definition of grouping sets in
`AggregateRel` to consist of references into a list of grouping
expressions instead of consisting of expressions directly. As discussed
in more detail in #700, the previous version is problematic because it
requires consumers to deduplicate these expressions, which, in turn,
requires to parse and understand 100% of these expression even in cases
where that understanding is otherwise optional. The new version avoids
that problem and, thus, allows consumers to be simpler.

Signed-off-by: Ingo Müller <ingomueller@google.com>

proto/substrait/algebra.proto

@@ -244,18 +244,30 @@ message AggregateRel {
   // Input of the aggregation
   Rel input = 2;
 
-  // A list of one or more grouping expression sets that the aggregation measures should be calculated for.
-  // Required if there are no measures.
+  // A list of zero or more grouping sets that the aggregation measures should
+  // be calculated for. There must be at least one grouping set if there are no
+  // measures (but it can be the empty grouping set).
   repeated Grouping groupings = 3;
 
   // A list of one or more aggregate expressions along with an optional filter.
   // Required if there are no groupings.
   repeated Measure measures = 4;
 
+  // A list of zero or more grouping expressions that grouping sets (i.e.,
+  // `Grouping` messages in the `groupings` field) can reference. Each
+  // expression in this list must be referred to by at least one
+  // `Grouping.expression_reference`.
+  repeated Expression grouping_expressions = 5;
+
   substrait.extensions.AdvancedExtension advanced_extension = 10;
 
   message Grouping {
-    repeated Expression grouping_expressions = 1;
+    // Deprecated in favor of `expression_reference` below.
+    repeated Expression grouping_expressions = 1  [deprecated = true];
+
+    // A list of zero or more references to grouping expressions, i.e., indices
+    // into the `grouping_expression` list.
+    repeated uint32 expression_reference = 2;
   }
 
   message Measure {