Provide a REST API to launch an asynchronous conversion

Launch an asynchronous by doing a POST on the @convert adapter, with the parameter async to true:

POST http://localhost:8080/nuxeo/api/v1/path/to/file/@convert?async=true

Returns a JSON object with the following data:

{
  "entity-type": "conversionScheduled",
  "conversionId": "id",
  "pollingURL": "http://localhost:8080/nuxeo/api/v1/conversion/id/poll"
}

New endpoint available to poll and get the result of a conversion:

GET http://localhost:8080/nuxeo/api/v1/conversion/id/poll

For conversion not completed, returns the status of the conversion with a 200 HTTP code:

{
  "entity-type": "conversionStatus",
  "conversionId": "id",
  "status": "running" // scheduled, completed
}

For conversion completed, returns the result URL in the Location header with a 303 HTTP code.

GET http://localhost:8080/nuxeo/api/v1/conversion/id/result

Returns the result Blob of the conversion if any, returns 404 if there is no conversion / no result yet for the conversion.