A package with APIs to retrieve topic names and QoS settings

[Yadunund]

Presently, the names for various internal ROS 2 topics are defined in StandardNames.hpp files in multiple packages such as rmf_fleet_adapter, rmf_traffic_ros2, rmf_task_ros2.

Downstream packages need to depend on these packages to include this header file to retrieve topic names even though they do not need any other artifacts. But even so, the correct DDS QoS settings for the topic is not apparent.

Proposal:
We define a separate package, rmf_internal_interfaces (or something relevant)

Then inside a single header file, say RMFInternalInterfaces.hpp:

For each internal interface, say FleetStates that is meant to provide information to publish and subscribe to a topic carrying rmf_fleet_msgs::msg::FleetState message, we have

namespace FleetStates {

  static std::string topic_name() { return "/fleet_states"; }
  static rclcpp::QoS qos_profile() { return rclcpp::QoS(10).reliable(); }
  
} // namespace FleetStates

such that downstream applications can

_fleet_states_pub = node->create_publisher(
  rmf_internal_interfaces::FleetStates::topic_name(),
  rmf_internal_interfaces::FleetStates::qos_profile()
);

Instead of namespaces we could also define structs.

For stricter API control, we could also have a pure abstract base class

class InterfaceBase
{
public:
  virtual std::string topic_name() const = 0;
  virtual rclcpp::QoS qos_profile() const = 0;
  virtual ~InterfaceBase() = default;
}

which can be implemented

class FleetStates : public InterfaceBase
{
public:
  std::string topic_name() const final { return "/fleet_states"; }
  rclcpp::QoS qos_profile() const final { return rclcpp::QoS(10).reliable(); }
} 

But the latter would require users to instantiate objects of these classes first.