Change JSON serialization format: use type as discriminator field

[grzegorz-orbital]

The JSON serialization format should be updated so that structures use a discriminator field named type.
Instead of nesting variant data under the variant name, the variant should be expressed using a top-level type field.

Example before:

{
    "variant": {
        "field1": "string",
        "field2": 123
    }
}

Example after:

{
    "type": "variant",
    "field1": "string",
    "field2": 123
}