Prefill/default values for items in repeating groups/lists

### Description

We would like to be able to prefill/initialize items in repeating groups/lists with default/prefilled values (eg. country code for a list of contact persons). Today's solution is to iterate through the list in DataProcessor, making the experience for the end user less than desirable given the need to trigger a change in order for the prefill value to be set, for each new item in the list.

### Additional Information

Given a `List` one solution could utilize default property value in the data model, like:
```CSharp
public class ContactPerson
{
public string Name { get; set; }
public string PhoneNumber { get; set; }
// Initializes PhonePrefix with a default value of "+47"
public string PhonePrefix { get; set; } = "+47";
}
```
As of today, this will not be written to instance data. Another solution could be to enable "defaultValue" on components.

Start by tracing how DataProcessor creates or initializes new items in repeating groups/lists, then compare that path with model property defaults and component defaultValue handling. Done means agreeing on one supported prefill approach and ensuring values such as a ContactPerson PhonePrefix are written to instance data without a user-triggered change.

Written by the indexing model from the issue text.