Add RequestParsingMiddleware for Simplified Request Body Parsing (Fixes #3369)

[Nirab123456]

This pull request introduces the RequestParsingMiddleware to the Tornado framework, addressing the concerns raised in issue #3369 regarding the need to manually parse POST request bodies. The middleware provides automatic parsing of request bodies based on the Content-Type header, supporting JSON, form-encoded data, and multipart form data (including file uploads).

---

Key Changes:

1. RequestParsingMiddleware:

   • Automatically parses request bodies when the Content-Type is either:
     • application/json
     • application/x-www-form-urlencoded
     • multipart/form-data
   • Sets the parsed data to handler.parsed_body, making it easier to access body data without manual processing.
   • Handles both regular form data and file uploads for multipart requests.

2. Test Coverage:

   • Added unit tests to verify that RequestParsingMiddleware handles various content types correctly:
     • test_json_parsing: Verifies JSON body parsing.
     • test_form_parsing: Validates form-encoded body handling.
     • test_multipart_parsing_with_file: Tests multipart form data parsing, including file uploads.

---

Example Usage:

This middleware can be easily integrated into Tornado applications. Below is an example of how a handler uses RequestParsingMiddleware to automatically parse incoming POST requests:

import tornado.ioloop
import tornado.web
from tornado.webmiddleware import RequestParsingMiddleware

class MainHandler(tornado.web.RequestHandler):
    def prepare(self):
        self.parsed_body = None
        self._apply_middlewares()

    def _apply_middlewares(self):
        middlewares = [RequestParsingMiddleware()]
        for middleware in middlewares:
            middleware.process_request(self)

    def post(self):
        self.write(self.parsed_body)

def make_app():
    return tornado.web.Application([
        (r"/", MainHandler),
    ])

if __name__ == "__main__":
    app = make_app()
    app.listen(8888)
    tornado.ioloop.IOLoop.current().start()

---

Issue Fixed:

• Fixes tornado.httputil.HTTPServerRequest and tornado.web.RequestHandler  #3369: This middleware removes the need for developers to manually parse request bodies in Tornado's RequestHandler, especially when dealing with various content types. This also prevents MissingArgumentError when accessing body arguments directly.