feat: add hint containing field names to RelCommon

Allows the output field names of any relation to be set or reset

Signed-off-by: andrew-coleman <andrew_coleman@uk.ibm.com>

proto/substrait/algebra.proto

@@ -41,6 +41,8 @@ message RelCommon {
     // Name (alias) for this relation. Can be used for e.g. qualifying the relation (see e.g.
     // Spark's SubqueryAlias), or debugging.
     string alias = 3;
+    // Sets (or resets) the output field names for any relation.
+    repeated string output_names = 4;
 
     substrait.extensions.AdvancedExtension advanced_extension = 10;