Add support for callbacks

[samchungy]

example openapi

{
  "openapi": "3.0.0",
  "info": {
    "title": "Callback Example",
    "version": "1.0.0"
  },
  "paths": {
    "/streams": {
      "post": {
        "description": "subscribes a client to receive out-of-band data",
        "parameters": [
          {
            "name": "callbackUrl",
            "in": "query",
            "required": true,
            "description": "the location where data will be sent.  Must be network accessible\nby the source server\n",
            "schema": {
              "type": "string",
              "format": "uri",
              "example": "https://tonys-server.com"
            }
          }
        ],
        "responses": {
          "201": {
            "description": "subscription successfully created",
            "content": {
              "application/json": {
                "schema": {
                  "description": "subscription information",
                  "required": [
                    "subscriptionId"
                  ],
                  "properties": {
                    "subscriptionId": {
                      "description": "this unique identifier allows management of the subscription",
                      "type": "string",
                      "example": "2531329f-fb09-4ef7-887e-84e648214436"
                    }
                  }
                }
              }
            }
          }
        },
        "callbacks": {
          "onData": {
            "{$request.query.callbackUrl}/data": {
              "post": {
                "requestBody": {
                  "description": "subscription payload",
                  "content": {
                    "application/json": {
                      "schema": {
                        "type": "object",
                        "properties": {
                          "timestamp": {
                            "type": "string",
                            "format": "date-time"
                          },
                          "userData": {
                            "type": "string"
                          }
                        }
                      }
                    }
                  }
                },
                "responses": {
                  "202": {
                    "description": "Your server implementation should return this HTTP status code\nif the data was received successfully\n"
                  },
                  "204": {
                    "description": "Your server should return this HTTP status code if no longer interested\nin further updates\n"
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Originally posted by @kambie10 in #272