Fix: Unauthorized Access Error For PAR (#671)

### Changes

- Fix for  `401: Unauthorized `for PAR Requests


### Contributor Checklist

- [x] I agree to adhere to the [Auth0 General Contribution
Guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md).
- [x] I agree to uphold the [Auth0 Code of
Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md).

Author: arpit-jn

auth0/authentication/pushed_authorization_requests.py

@@ -3,6 +3,7 @@
 from .base import AuthenticationBase
 
 
+
 class PushedAuthorizationRequests(AuthenticationBase):
     """Pushed Authorization Request (PAR) endpoint"""
 
@@ -24,9 +25,11 @@ def pushed_authorization_request(
         return self.authenticated_post(
             f"{self.protocol}://{self.domain}/oauth/par",
             data={
-                "client_id": self.client_id,
+                "client_id":self.client_id,
+                "client_secret":self.client_secret,
                 "response_type": response_type,
                 "redirect_uri": redirect_uri,
                 **kwargs,
             },
-        )
+            headers={"Content-Type": "application/x-www-form-urlencoded"},
+        )

auth0/rest.py

@@ -7,6 +7,7 @@
 from random import randint
 from time import sleep
 from typing import TYPE_CHECKING, Any, Mapping
+from urllib.parse import urlencode
 
 import requests
 
@@ -152,6 +153,12 @@ def _request(
         # Reset the metrics tracker
         self._metrics = {"retries": 0, "backoff": []}
 
+        if data is None and json is not None and headers:
+            content_type = headers.get("Content-Type", "").lower()  # Get Content-Type 
+            if "application/x-www-form-urlencoded" in content_type:
+                data = urlencode(json)  # Copy JSON data into data
+                json = None  # Prevent JSON from being sent
+
         kwargs = {
             k: v
             for k, v in {