The wrapper is defined in the method _json_response of class JsonRequest.

You can rewrite this method for the object "request" for your route only :

from odoo.http import request, JsonRequest, Response

from odoo.tools import date_utils

...

def alternative_json_response(self, result=None, error=None):

  if error is not None:

      response = error

  if result is not None:

      response = result

  mime = 'application/json'

  body = json.dumps(response, default=date_utils.json_default)

  return Response(

    body, status=error and error.pop('http_status', 200) or 200,

    headers=[('Content-Type', mime), ('Content-Length', len(body))]

  )

class YourController(http.Controller):

    ...

    @http.route ('/userappoint/webhook_test/', type = 'json', auth = 'public', method  = ['POST'], csrf = False, website = True)

    def webhook_test (self, ** kw):

       response = {

            'speech': 'hello my name is shubham',

            'displayText': 'hello testing',

            'source': 'webhook'

        }

        request._json_response = alternative_json_response.__get__(request, JsonRequest)

        return response