Implement the rmw_get_publishers_info_by_topic and rmw_get_subscriptions_info_by_topic methods

[jaisontj]

As a follow up to this PR which returns RMW_RET_UNSUPPORTED for the rmw_get_publishers_info_by_topic and rmw_get_subscriptions_info_by_topic; ensure that these methods are implemented before the ROS2 F release.

These methods are implemented in rmw_fastrtps in this PR which can be used as a reference.

[ivanpauno]

Assigning myself, same comment as ros2/rmw_opensplice#299 (comment).

[wjwwood]

When this is done, the tests need to be fixed in rclpy to actually test things:

https://github.com/ros2/rclpy/pull/454/files#diff-960e40f792ccd92281ee56dfb65726bcR197

[mm318]

The pull request for resolving this issue has been opened at #391.

[mm318]

When this is done, the tests need to be fixed in rclpy to actually test things:

https://github.com/ros2/rclpy/pull/454/files#diff-960e40f792ccd92281ee56dfb65726bcR197

This change is in pull request ros2/rclpy#511.

[clalancette]

Closing, since with Foxy now being End-of-Life, this repository is no longer used.