Add websockets support

[tribal-tec]

• All PUT messages are supported using a JSON text message with an "event"
  and "data" field where "event" corresponds to the REST endpoint and
  "data" as specified by endpoint/schema. For the camera, it looks like this:
  {
  "event" : "v1/camera",
  "data" : {
  "look_at" : [ 1.0 0.5 0.5 ]
  }
  }
• So far only the camera gets broadcasted as a JSON text message when it
  was modified. The JSON format is the same as for PUT, but contains all camera
  values.

[rolandjitsu]

@tribal-tec so we have but one shared socket for all events?

[rdumusc]

+1, nice and simple way to get started!
I was working in a branch in Rockets on moving the json-rpc parser from tide and adding asynchronous request processing, error codes, etc.. but it's likely not needed at this point. Interestingly I also ended up using this nlohmann::json :-)

[rolandjitsu]

I think there might be an issue with the {data}. By the looks of it, there seems to be double JSON serialization, on the main socket message and the {data} property:

{
    "data": "{\n   \"aperture\" : 0.0,\n   \"eye_separation\" : 0.063500002026557922,\n   \"field_of_view\" : 45.0,\n   \"focal_length\" : 0.0,\n   \"look_at\" : [ 0.73252975940704346, 1.1178562641143799, 1.0505576133728027 ],\n   \"origin\" : [ 2.442774772644043, 4.600367546081543, 7.2228507995605469 ],\n   \"stereo_mode\" : \"none\",\n   \"up\" : [ -0.45402175188064575, -0.82591044902801514, -0.33421853184700012 ]\n}\n",
    "event": "camera"
}

@tribal-tec

[tribal-tec]

True, good catch. Will provide a fix