Properly support `null | type` values in options

After JsonPatch support was implemented, our data models got support for non-string values, along with `null`, `number` and `boolean`. After support for non-string values in options was implemented in the backend (https://github.com/Altinn/app-lib-dotnet/issues/413), some issues have been uncovered:

**Support for `oneOf`/`anyOf` in data model schema**
Our current solution that resolves data model bindings in the JsonSchema to get the 'real' data type, and validate it, seems to not like more complex types, like the following:
```json
{
"Months": {
"oneOf": [
{ "type": "integer" },
{ "const": null }
]
}
}
```
When a component is bound to `Months` in this data model, it fails to render, saying the bound type is unsupported. Investigate what Studio creates, and support variants of this and `{ "type": "null" }`.

**Cast types with option-value restrictions**
Currently, we cast all options to strings in the `castOptionsToStrings()` function, so all option-components can assume their values are strings. However, there's a difference between `null` and `"null"`, so we should take care not to lose that distinction when casting the values back to their *real* counterparts. In `src/features/formData/convertData.ts` the form data system will try to cast the given value back to something that 'fits' in the data model, and it should possibly be the responsibility of `useGetOptions()` to cast option values before sending them to the form data system.

Start by investigating how Studio-created schemas with oneOf, anyOf, and null types are resolved for component bindings. Then trace castOptionsToStrings() and useGetOptions(), including src/features/formData/convertData.ts, to preserve the distinction between null and "null". Done means supported nullable option types render and option values are cast correctly for the data model.

Written by the indexing model from the issue text.