feat: add optional metadata containing field names to RelCommon (#696)

This PR introduces a new metadata field in the `RelCommon` hint definition which contains
the output field names for any relation. This enables full resolution roundtrip for `Expand` in the `spark` module.

Signed-off-by: Andrew Coleman <andrew_coleman@uk.ibm.com>

proto/substrait/algebra.proto

@@ -41,6 +41,9 @@ 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;
+    // Assigns alternative output field names for any relation.  Equivalent to the names field
+    // in RelRoot but applies to the output of the relation this RelCommon is attached to.
+    repeated string output_names = 4;
 
     substrait.extensions.AdvancedExtension advanced_extension = 10;