PostSimplified prefill does not support lists

### Description of the bug

When instantiating with prefill, the prefill ignores lists.


Looking into the code, it looks like the prefill have no list support implemented?
https://github.com/Altinn/app-lib-dotnet/blob/1be1b79ce004aaa27a8cd12fcfdba2bf6e839442/src/Altinn.App.Core/Implementation/PrefillSI.cs#L161

If this get fixed, it would be nice to also implement support for nested objects, instead of the current flat structure.

### Steps To Reproduce

Call PostSimplified (https://{org}.apps.altinn.no/{org}/{app}/instances/create) with prefill that contains a list that should be deserialized into a corresponding List class.

### Additional Information

_No response_

Start with the PrefillSI.cs code around line 161 and reproduce the issue through the PostSimplified instances/create endpoint using prefill containing a list. Trace how the prefill is deserialized, then verify that list values are populated in the corresponding model and consider the requested nested-object support; done means these cases are covered and tested.

Written by the indexing model from the issue text.