Add JSON Schema definition of the submitted data to the JSON registration backend plugin

[viktorvanwijk]

Sub task of #4908

Example

An example form with the following configuration:

• Authentication via DigiD, capturing a BSN
• First form step with text fields firstName and lastName
• Second form step with a user upload attachment

Should lead to a JSON object being sent with the shape:

{
    "values":
    {
        "auth_bsn": "123456782",
        "firstName": "Donnie",
        "lastName": "Darko",
        "attachment": "<base64 encoded data>"
    },
    "schema":
    {
        "$schema": "https://json-schema.org/draft/2020-12/schema",
        "type": "object",
        "properties":
        {
            "auth_bsn":
            {
                "type": "string",
                "pattern": "^\\d{9}$"
            },
            "firstName":
            {
                "type": "string"
            },
            "lastName":
            {
                "type": "string"
            },
            "attachment":
            {
                "type": "string",
                "contentEncoding": "base64"
            }
        },
        "required":
        [
            "auth_bsn",
            "firstName",
            "lastName"
        ],
        "additionalProperties": false
    }
}