api: change the `GET /v1/bricks/{id}` variable field

[dido18]

motivation

Align the variable configuration of those refactored here #18

These endpoints return a config_variables as an array.

• GET /v1/apps/{id}/bricks
• GET /v1/apps/{id}/bricks/{id}

Change the

• GET /v1/bricks/{id}

changes

 curl 127.0.0.1:8800/v1/apps/ZXhhbXBsZXM6dmlkZW8tZmFjZS1kZXRlY3Rpb24/bricks
{
  "id": "arduino:object_detection",
  "name": "Object Detection",
  "author": "Arduino",
  "description": "n",
  "category": "video",
  "status": "installed",

// DEPRECATE the variables
  "variables": {
    "CUSTOM_MODEL_PATH": {
      "default_value": "/home/arduino/.arduino-bricks/ei-models",
      "description": "path to the custom model directory",
      "required": false
    },
    "EI_OBJ_DETECTION_MODEL": {
      "default_value": "/models/ootb/ei/yolo-x-nano.eim",
      "description": "path to the model file",
      "required": false
    }
  },
 // Refactor and use the "config_variable"
   "config_variables": [
  {
  "name": "CUSTOM_MODEL_PATH",
  "default_value": "/home/arduino/.arduino-bricks/ei-models",
    "description":"",
    "required": true
}
]