Expose --headless-rendering as a sdf tag to gazebo server plugin

[adityapande-1995]

Desired behavior

The --headless-rendering command line flag was recently added to ignition CLI (https://github.com/ignitionrobotics/ign-gazebo/blob/77dd8551e942c6f2a9dfbd6360a51cab9185ebab/src/cmd/cmdgazebo.rb.in#L76) , where one can say : ign gazebo -s --headless-rendering my_world.sdf. This has not been ported to GazeboServer.cc in the gazebo server plugin for ignition-launch. It would be nice to have a sdf tag expose that flag :

<plugin name="ignition::launch::GazeboServer"
            filename="ignition-launch-gazebo">
      <world_file>my_world.sdf</world_file>
      <headless_rendering>true</headless_rendering>
    </plugin>

Alternatives considered

The only way to do this now using ignition launch is to use the <executable> tag :

<executable name='gazebo-server'>
    <command>ign gazebo  -s --headless-rendering my_world.sdf</command>
</executable>