Make joint_states subscription QoS configurable; default to SensorDat…

…aQoS. (#179)

This changes the subscription default from `reliable` to
`best_effort`. This is helpful when visualizing a robot that's running
on a wifi-connected device, and eliminates a warning when starting up
drivers on a robot that publishes with `best_effort`.

The subscription defaults to `best_effort`, but can be overridden with
the following config:

```yaml
robot_state_publisher:
  ros__parameters:
    qos_overrides:
      /joint_states:
        subscription:
          reliability: reliable
```

src/robot_state_publisher.cpp

@@ -136,11 +136,16 @@ RobotStatePublisher::RobotStatePublisher(const rclcpp::NodeOptions & options)
 
   setupURDF(urdf_xml);
 
+  auto subscriber_options = rclcpp::SubscriptionOptions();
+  subscriber_options.qos_overriding_options =
+    rclcpp::QosOverridingOptions::with_default_policies();
+
   // subscribe to joint state
   joint_state_sub_ = this->create_subscription<sensor_msgs::msg::JointState>(
-    "joint_states", 10, std::bind(
-      &RobotStatePublisher::callbackJointState, this,
-      std::placeholders::_1));
+    "joint_states",
+    rclcpp::SensorDataQoS(),
+    std::bind(&RobotStatePublisher::callbackJointState, this, std::placeholders::_1),
+    subscriber_options);
 
   publishFixedTransforms();