Sub-form response that allows multiple responses

Nextcloud (please complete the following information):

• Nextcloud-Version: 26.0.x
• Forms-Version: 3.3.x

Is your feature request related to a problem? Please describe.
I need the ability to construct a response that I would describe as a sub-form that allows the user to add an arbitrary number of responses to a list. For example, a travel form that allows the user to make an itinerary of stops

Describe the solution you'd like
The feature would allow the user to design a form and then use that form as sub-form. In the travel example, I would make a travel stop form that has an arrival date, departure date, city, country.

The main form would have a list element that would allow the user to add an arbitrary number of stops to the itinerary element of the travel form.

Ideally, the form designer could designate an element in the sub-form as the sort element that would sort the responses within the list.

Describe alternatives you've considered
A large text block; however, there is no way to enforce formatting and field validation (e.g. valid dates).

No files, tests, or entry points are identified in the issue. Start by locating the form-builder and response-handling entry points in nextcloud/forms, then define the sub-form, repeatable-list, validation, and sorting behavior; done means designers can embed a sub-form and users can add and order arbitrary responses.