pop-up the list of existing entity types when defining a task element in the FogFlow dashboard

[showersky]

When we use the FogFlow dashboard to specify the task element of a service topology or a fog function, we need to define the entity type of its input data. Currently this must be manually written by the user, it will be convenient to the user if we can pop up a drop-down menu for the user to directly select one of the existing entity types. The list of existing entity types can be retrieved from Discovery or from the designer (via Dgraph). Maybe we could add one specify API to provide this list.