Load middleware and phases from `middleware.json`

Sample JSON:

        {
          "routes:before": {
            "morgan": {
              "params": ["dev"]
            }
          },
          "routes": {
            "loopback/server/middleware/rest": {
            }
          },
          "subapps": {
            "./adminer": {
            },
          }
        }

The JSON file can be customized using the usual conventions:
  - middleware.local.{js|json}
  - middleware.{env}.{js|json}

It is also possible to mount the same middleware in the same phase
multiple times with different configuration.

Example config:

    {
      "auth": {
        "oauth2": [
          {
            "params": "first"
          },
          {
            "params": "second"
          }
        ]
      },
    });