Add rmw listener apis

[irobot-ros]

This PR adds stub implementations for the RMW listener APIs.
See design and Discourse post.

FYI @wjwwood

This PR connects to

• design: Events Executor design design#305
• rclcpp: Add RMW listener APIs rclcpp#1579
• rcl: Add Events Executor rcl#839
• rmw: Add EventsExecutor rmw#286
• rmw_implementation: Add EventsExecutor rmw_implementation#161
• rmw_fastrtps: Add EventsExecutor rmw_fastrtps#468
• rmw_cyclonedds: Add EventsExecutor rmw_cyclonedds#256

[asorbini]

Hi @irobot-ros, thank you for this contribution!

The code looks good functionally, but I'd like to request some minor refactoring to make it more consistent in style with the rest of the implementation:

• Use UNUSED_ARG(var); instead of (void)var;
• Use RMW_CONNEXT_LOG_ERROR_SET("...") (no semicolon) instead of RMW_SET_ERROR_MSG("...");
• Introduce a new source file in rmw_connextdds_common/src/common, e.g. rmw_listener.cpp, with a common implementation of all new API functions:
  • rmw_api_connextdds_event_set_callback()
  • rmw_api_connextdds_service_set_on_new_request_callback()
  • rmw_api_connextdds_client_set_on_new_response_callback()
  • rmw_api_connextdds_subscription_set_on_new_message_callback()
• Refactor functions in rmw_api_impl_ndds.cpp and rmw_api_impl_rtime.cpp to delegate to the common functions, similarly to other functions in those files.

[asorbini]

@mauropasse thank you for addressing the comments, and apologies for forgetting about this PR.

As is, it looks ready to merge as soon as ros2/rmw#286 goes in.