Submissions API call should return responses keyed by form block uuid

[PhilReinking]

Currently we get something like this:

{
        "id": 3540,
        "uid": "5c4SyRk0",
        "started_at": "2023-02-20 13:59:31",
        "completed_at": "2023-02-20 13:59:43",
        "params": null,
        "responses": [
            {
                "id": "pgWy",
                [...]
            },
            {
                "id": "qjWy",
                [...]
            }
        ]
}

To make that output actually usable in tools like Zapier/Make, responses should follow this structure:

{
        "id": 3540,
        "uid": "5c4SyRk0",
        "started_at": "2023-02-20 13:59:31",
        "completed_at": "2023-02-20 13:59:43",
        "params": null,
        "responses": {
            "pgWy": {
                [...]
            },
            "qjWy": {
                [...]
            }
        }
}