[feature_request] Add `franka_gazebo` physics engine argument

[rickstaa]

It would be beneficial if users could change the physics engine used by Gazebo.

I, for example, decided to use DART as the default physics engine since it appears to be the most stable (https://github.com/frankaemika/franka_ros/issues/160#issuecomment-989173918). The only downside of using this engine compared to the ODE engine is that the get_physics_properties service call still needs to support the physics engine [bullet].

I created https://github.com/frankaemika/franka_ros/pull/211 to add this launch file argument. This argument should also be documented in the documentation, but I need help finding the documentation repository.

[!IMPORTANT]\ Please note that the BULLET and SIMBODY physics engines do not work with the current model (see https://github.com/frankaemika/franka_ros/issues/160#issuecomment-989173918). I added a warning in https://github.com/frankaemika/franka_ros/pull/211 adds a warning to communicate this to the user.

TODOs

• [ ] Merge https://github.com/frankaemika/franka_ros/pull/211.
• [ ] Document in the documentation.