Support drop-down menu in ROS2

[HuaTsai]

Due to the ROS2 parameter descriptor, I think it is not easy to migrate the drop-down menu in ROS1.
My current workaround is to utilize the field additional_constraints in rcl_interfaces/msg/ParameterDescriptor.
We can add rules such as Options: a b c.

• Benefit: Enable drop-down menu for GUI
• Drawback: Restrict the usage of additional_constraints and users should know to edit it

I wonder how do you think of this workaround.