Return default value on getViewerJSON

[hvarg]

Is your feature request related to a problem? Please describe.
I'm using getViewerJSON to get workflow variable information, however, the response does not include the parameters default value.

Describe the solution you'd like
Add default value for variables to the response:

{
    "template": { ... },
    "inputs": [
        {
            "name": "InputTS",
            "options": [ ... ],
            "dim": 0,
            "dtype": "ontology.owl#InputType",
            "id": "some_id",
            "type": "data",
            "default": "default_value"    // <-- This should be added
        },
        ...
    ]
}

Additional context
Used to display default value on DISK UI.