Library API: Create reference form-js form that includes every element in the BDT.tSituation data type

Ideally, this could be automatically generated, but might need to build this out manually at first.

This would be a place to work out how every possible input element could be constructed, including any frontend tricks to get things to play nicely with the canonical BDT.tSituation data type (implemented in BDT.dmn)

Delivery idea: create a page to serve the reference form on the library api (e.g. `/formtest`) and include facilities to submit the form to any of the running benefits in the API.

Uses:
- easily test various benefits
- research for how we might auto-generate various form elements in the builder (eventually)

Inizia individuando l’API della libreria e l’implementazione BDT.dmn del tipo di dati BDT.tSituation. Esamina come l’API serve le pagine e accetta gli invii, quindi definisci il modulo di riferimento in /formtest in modo che copra ogni possibile elemento di input e possa inviare dati a benefits in esecuzione.

Scritto dal modello di indicizzazione a partire dal testo della issue.