Support Spanner's UNION syntax, which needs to be UNION DISTINCT or UNION ALL. (#101768)

* Support Spanner's UNION syntax, which needs to be UNION DISTINCT or UNION ALL.

pkg/services/sqlstore/migrator/spanner_dialect.go

@@ -325,3 +325,7 @@ func confToClientOptions(connectorConfig spannerdriver.ConnectorConfig) []option
 	}
 	return opts
 }
+
+func (s *SpannerDialect) UnionDistinct() string {
+	return "UNION DISTINCT"
+}