[ROS2] add -J option to spawn_entity.py node

[millerluki]

Description

In ROS1 it was possible to spawn a crane with different joint values from command line or launch file with the option -J. This feature is missing in ROS2.

For example from command line:
In ROS1 it runs with
rosrun gazebo_ros spawn_model -urdf -param robot_description -J joint_name 0.7

To Reproduce

see above

Expected behavior

Add argument '-J' to set joint position when spawning the model. In ROS2 it could be similar like this
ros2 run gazebo_ros spawn_entity.py -topic robot_description -J joint_name 0.7

Screenshots

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Environment (please complete the following information):

• OS: Ubuntu 20.04
• Gazebo version: Gazebo 11
• ROS version: Foxy
• gazebo_ros_pkgs: apt install ros-foxy-gazebo-ros-pkgs
• glx server in use: output from glxinfo | grep 'server glx vendor' (glxinfo is in mesa-utils package for Debian/Ubuntu)

Are there any plans to port this feature to Gazebo11/ROS2 in the next time?