Add a new api to return all outputs for a cell

[3coins]

The current outputs service only provides an api for fetching each outputs individually. During initial loads for a notebook, for cells that have more than one outputs, this causes multiple fetch calls increasing network traffic and potentially delaying hydrating the outputs for the notebook. Outputs service should provide a new api to return all outputs for the cell in a single request.

Expected request url

/api/outputs/{file_id}/{cell_id}

Expected response

[
  {
    "output_type": "stream",
    "text": "Hello World 1"  
  }, 
  {
    "output_type": "stream",
    "text": "Hello World 2"
  }
]

[ellisonbg]

Do you think it should be a nested JSON object or is a bunch of JSON lines ok? The reason I am asking it that the outputs are JSON serialized on disk. Building the above nested object will require extra deserialization of each output and then revelation of the entire thing. Using JSON lines will avoid the de/serialization on the server

[3coins]

@ellisonbg
JSON lines will be probably better, but have to look more into how to handle them on the front-end.