New API to submit a form from an end user system (EUS) in one operation

To be able to scale for form services with high volumes and EUS as the primary channel (e.g. the Tax Return form for businesses) it's necessary to be able to submit the form with less API calls than in the current Altinn 3 solution.

There should be an API similar to soap and rest APIs in Altinn 2 where an EUS can create an instance, upload all forms and attachments, complete the workflow and get a receipt in one single API call.

The first version could be a wrapper using existing APIs so that EUSes quickly can start to adopt the new API. The final version should be internally optimized to minimize processing to complete the work.

Start by reviewing the existing Altinn 3 APIs and the analogous Altinn 2 SOAP and REST APIs mentioned in the issue. Define the scope and entry points for a single operation that creates an instance, uploads forms and attachments, completes the workflow, and returns a receipt; no files or tests are named.

Written by the indexing model from the issue text.