refactor: add default value for eggress remote 'qos' and 'retain'

otherwise when updating from dashboard, there is no way to set
'false' for 'retain' because it's a checkbox,
it's either 'true' or 'undefined'

apps/emqx_connector/src/mqtt/emqx_connector_mqtt_schema.erl

@@ -255,7 +255,8 @@ fields("egress_remote") ->
             mk(
                 qos(),
                 #{
-                    required => true,
+                    required => false,
+                    default => 1,
                     desc => ?DESC("egress_remote_qos")
                 }
             )},
@@ -263,7 +264,8 @@ fields("egress_remote") ->
             mk(
                 hoconsc:union([boolean(), binary()]),
                 #{
-                    required => true,
+                    required => false,
+                    default => false,
                     desc => ?DESC("retain")
                 }
             )},