simplify manifest/response spec

[kevinykuo]

@javierluraschi @anchit-sadana realized we could simplify the json specs a bit, lmk if objection

proposed:

{
    "manifests": [
        {
            "runtime": "r",
            "calls": [
                {
                    "node": "species_dropdown",
                    "fn_name": "on_select",
                    "args": [
                        {
                            "name": "value",
                            "value": "setosa"
                        }
                    ]
                },
                {
                    "node": "summary_table",
                    "fn_name": "html",
                    "args": []
                }
            ]
        }
    ]
}

old:

{
    "manifests": [
        {
            "runtime": "r",
            "nodes": [
                {
                    "name": "species_dropdown",
                    "calls": [
                        {
                            "fn_name": "on_select",
                            "args": [
                                {
                                    "name": "value",
                                    "value": "setosa"
                                }
                            ]
                        }
                    ]
                },
                {
                    "name": "summary_table",
                    "calls": [
                        {
                            "fn_name": "html",
                            "args": []
                        }
                    ]
                }
            ]
        }
    ]
}

[anchit-sadana]

Looks good to me