Why form.change can't change value if there is not a corresponding field ?

Hello !
i tried to change my form values using form.change, but it seems to work only if there is a corresponding field. Is there any reason to not allow to change freely values and add new keys ?

My use case : i have a "country" autocomplete text fields.
When a "country" is selected, autocomplete give me a complex object about the country like this
```js
{
name: "France",
code: "FR",
...otherProperties
}
```

and i would like to add this to my submitted values. Do not care so much about what there is in my autocomplete input, i cared much about the object, return by the autocomplete suggestion list.

For now i am playing with "format" prop and return an object as a value on onChange, but it feels somehow hackish to me ?
```
suggestion.label}
className="is-large"
autocompleteCallback={getCountriesByName}
/>
```

Start by tracing the form.change entry point and the Field value flow, including the format and onChange workaround shown in the issue. Determine the intended behavior for values without a corresponding field; done means the country object use case has a clear supported behavior and regression coverage.

Written by the indexing model from the issue text.