feat: adds flutter sdk template in connector config (#8438)

Co-authored-by: joehan <joehanley@google.com>

Author: OutdatedGuy

templates/init/dataconnect/connector.yaml

@@ -12,3 +12,6 @@ connectorId: __connectorId__
 #   kotlinSdk:
 #     outputDir: <Path where you want the generated SDK to be written to, relative to this file>
 #     package: connectors.default
+#   dartSdk:
+#     outputDir: <Path where you want the generated SDK to be written to, relative to this file>
+#     package: default_connector