Run Gazebo and ros_gz_bridge as ROS components in the same process

[azeey]

Complementary to #490

Desired behavior

Currently ros_gz_bridge provides a ROS node that runs as a separate process. This means, both Gazebo<-> ros_gz and ros_gz<-> ROS incur the cost of serialization and network transport.

Alternatives considered

#490

Implementation suggestion

With #500 we're adding a ROS node that starts Gazebo instead of using the gz sim CLI. This node and ros_gz_bridge can be turned into a ROS components that can be run in the same container (same process).

ros_gz_image and ros_gz_pointcoud can be reimplemented or deprecated since the were created to overcome the serialization cost of image and pointcloud transport.