Offer asynchronous model upload/import

[ghost]

This comes from a short discussion with @aedelmann.
Relates to some existing closed issues such as #1667 and #1997.

The idea would be to either offer new endpoints, or modify the existing ones in order to provide an asynchronous model upload and import functionality.

Expected features:

• Both POST and PUT endpoints would return immediately with a JSON message containing a token

• One or two new GET endpoints would be offered where the token is initially retrieved by the client is the path resource or a parameter

• The GET endpoint(s) would provide a synchronous JSON response with details regarding:

  • Progress if possible
  • Status (processing, success, error / timeout)
  • Error / timeout details if applicable

The safest way would be to provide the new POST endpoints in alternative to the existing ones, so existing clients outside Vorto's front-end would not be impacted.

Vorto's front-end would leverage this to provide some meaningful information to the end-user, while their model is getting uploaded.

This could also be a good occasion to change the upload/import process from two steps to a one-step upload+import combination.

The latter change might even be considered as a one-step upstream, meaning one POST endpoint for uploading and importing in one go.

@aedelmann give me a shout if you agree / disagree / have ideas to share!