[rmf-dashboard] Support for /robot_task for task form

[youliangtan]

Using the task submission form below as an example. Currently it is not possible to dispatch a specific robot to a shared robot with the current task form (dispatch a Loop task to RobotX from FleetY, to waypointZ), this is not possible with the current task form

Since we already have the REST api /tasks/robot_task (POST method) ready in the api server, the solution is merely adding 2 input boxes on the GUI, and submit the json task to /tasks/robot_task from the client.