Add `/download/json` API endpoint for downloading full document as JSON

Describe the problem to be solved

I'm quite new to Grist so maybe i haven't seen this, but i can't find a way to download all data from a single document as a JSON file. Especially for smaller setups and PoC work it would be very useful. Currently the API supports downloading a document either as a SQLite or Excel file. The only other way currently is writing a script that iterates over all the /tables endpoints of a document.

Describe the solution you would like

Add a /download/json endpoint for documents, e.g

curl http://localhost:8484/o/docs/api/docs/<id>/download/json

Would give something very similar to going over all /tables endpoints individually.