Enable ROS2 service calls from C++ nodes

[phil-opp]

Allows creating ROS2 service clients in C++ nodes using our dora-ros2-bridge.

Builds upon #439

Usage

• Initialize the ROS2 context using init_ros2_context()
• Create ROS2 node using ros2_context->new_node
• Define QoS settings for service, e.g.:

  auto service_qos = qos_default();
  service_qos.reliable = true;
  service_qos.max_blocking_time = 0.1;
  service_qos.keep_last = 1;

• Create service client using one of the node->create_client_* functions. For example, to create a client to the add_two_ints service:

  auto add_two_ints = node->create_client_example_interfaces_AddTwoInts("/", "add_two_ints", service_qos, merged_events);

  The merged_events argument is the combined event stream that should receive the response data.
• Wait until the service is available using: add_two_ints->wait_for_service(node)
• Send one or multiple requests using add_two_ints->send_request(request). The responses will be sent to the combined event stream.
• To check if an incoming event is a service response, use the matches and downcast functions:

  if (add_two_ints->matches(event))
  {
      auto response = add_two_ints->downcast(std::move(event)); 
      ...
  }

[haixuanTao]

Great thanks!

[phil-opp]

Thanks @bobd988 for the README update!