New IDataProcessing interface that supports multiple DataElements and adding elements in response to changes

[ivarne]

Description

With support for subforms and multiple data models and the multiple PATCH endpoint, app developers want to read and write multiple data elements in a single request, and also to be able to ADD subform instances based on information changes in the main data element.

Deleting is different, because the current PATCH interface does not allow to signal deletion, and in addition it is likely better to have some sort of hidden mechanism so that the data isn't actually deleted until we trigger /process/next

Additional Information

Frontend will not yet support receiving new data elements in a PATCH response, and this needs to be added. If we want to support deletion of whole data elements, an appropriate interface needs to be negotiated. Likely solution would be to send instance.Data (or the full instance object)