Frontend: Implement updated data model selection design (begind feature flag)

[nkylstad]

Description

For apps with existing config (feature flag not activated), when opening data model binding for edit:

• Show selected data model (default data model from layoutset)
• Show data model field select as usual (no change)

For apps with new config (or when feature flag is activated), when opening data model binding for edit:

• Show all available data models in a select/combobox. Default model should be selected if config does not have datamodel specified.
• Allow user to select data model
• Select for data model fields should reflect the fields from the selected data model.
• Updating data model binding should set the new structure, where both field and data model are specified.

[ghost]

Should also add a Playwright test for this 😄

[Annikenkbrathen]

Design suggestion for apps with existing config @lassopicasso
Show selected data model (default data model from layoutset)

I suggest to use the Altinn Studio component for a readonly field that is not editable.

[JamalAlabdullah]

Tested ok in dev