Added proxy headers option for urllib3 (#2467)

* adding headers option to pass in rest client

* running ensure uptodate

samples/client/petstore/python-asyncio/petstore_api/configuration.py

@@ -98,6 +98,8 @@ class Configuration(six.with_metaclass(TypeWithDefault, object)):
 
         # Proxy URL
         self.proxy = None
+        # Proxy headers
+        self.proxy_headers = None
         # Safe chars for path_param
         self.safe_chars_for_path_param = ''
         # Adding retries to override urllib3 default value 3