Fixes #228

Pydantic v2 .__dict__ has different behavior from what Pydantic v1 had and is not giving us `extra` fields anymore. On the other hand the iterator over the event gives us extras as well

Signed-off-by: Yurii Serhiichuk <savik.ne@gmail.com>

cloudevents/pydantic/v2/event.py

@@ -209,7 +209,7 @@ def _ce_json_dumps(self) -> typing.Dict[str, typing.Any]:
     def _get_attributes(self) -> typing.Dict[str, typing.Any]:
         return {
             key: conversion.best_effort_encode_attribute_value(value)
-            for key, value in self.__dict__.items()
+            for key, value in dict(BaseModel.__iter__(self)).items()
             if key not in ["data"]
         }