feat: allow naming/aliasing relations (#649)

Same goal as in #648 - to support what Spark's and DataFusion's
SubqueryAlias relation does.

As Substrait mostly only referes to columns by their index, there is no
inherent need for table name/qualifiers within Substrait. However, some
consumers, e.g. DataFusion, require column names to be either unique or
qualified for joins, which is troublesome w/o the possibility to qualify
relations.

Also, for debugging failed plans and for roundtrip testing of X ->
Substrait -> X conversions, it would be convenient to have proper,
human-readable names to refer to.

Closes #571

proto/substrait/algebra.proto

@@ -37,6 +37,11 @@ message RelCommon {
   message Hint {
     Stats stats = 1;
     RuntimeConstraint constraint = 2;
+
+    // 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;
+
     substrait.extensions.AdvancedExtension advanced_extension = 10;
 
     // The statistics related to a hint (physical properties of records)