[uxrce_dds_client] Allow for arbitrary topic instances to be bridged (#22350)

Signed-off-by: Beniamino Pozzan <beniamino.pozzan@gmail.com>
Co-authored-by: Jacob Dahl <37091262+dakejahl@users.noreply.github.com>

docs/en/middleware/uxrce_dds.md

@@ -503,6 +503,11 @@ publications:
   - topic: /fmu/out/vehicle_trajectory_waypoint_desired
     type: px4_msgs::msg::VehicleTrajectoryWaypoint
 
+  - topic: /fmu/out/vehicle_imu
+    type: px4_msgs::msg::VehicleImu
+    rate_limit: 50.
+    instance: 1 # OPTIONAL
+
 subscriptions:
 
   - topic: /fmu/in/offboard_control_mode
@@ -535,6 +540,9 @@ Each (`topic`,`type`) pairs defines:
 4. The message type (`VehicleOdometry`, `VehicleStatus`, `OffboardControlMode`, etc.) and the ROS 2 package (`px4_msgs`) that is expected to provide the message definition.
 5. **(Optional)**: An additional `rate_limit` field (only for publication entries), which specifies the maximum rate (Hz) at which messages will be published on this topic by PX4 to ROS 2.
    If left unspecified, the maximum publication rate limit is set to 100 Hz.
+6. **(Optional)**: An additional `instance` field (only for publication entries), which lets you select which instance of a [multi-instance topic](./uorb.md#multi-instance) you want to be published to ROS 2.
+   If provided, this option changes the ROS 2 topic name of the advertised uORB topic appending the instance number: `fmu/out/[uorb topic name][instance]` (plus eventual namespace and message version).
+   In the example above the final topic name would be `/fmu/out/vehicle_imu1`.
 
 `subscriptions` and `subscriptions_multi` allow us to choose the uORB topic instance that ROS 2 topics are routed to: either a shared instance that may also be getting updates from internal PX4 uORB publishers, or a separate instance that is reserved for ROS2 publications, respectively.
 Without this mechanism all ROS 2 messages would be routed to the _same_ uORB topic instance (because ROS 2 does not have the concept of [multiple topic instances](../middleware/uorb.md#multi-instance)), and it would not be possible for PX4 subscribers to differentiate between streams from ROS 2 or PX4 publishers.