Persistence API to support JSON for form data

[ebruchez]

At least for GET, maybe for PUT.

See also #2109.

+1 from evaluator

[ebruchez]

+1 from customer today

[ebruchez]

Form definitions, on the other hand, do not follow the same format as regular form data. In addition, it doesn't make sense to convert them to JSON. Options:

• keep them as XML and store them like before
• keep them as XML and wrap them into a simple JSON string or object
• have a separate table just for Form Builder form data
• have a separate column just for Form Builder form data
• anything else?

Note that with some databases, the JSON must be stored in a separate "table" or collection.

From the CRUD API point of view, data could be PUT with application/json, and form definitions PUT with application/xml.

Q: Should the persistence proxy do the XML <-> JSON conversion?