email: add Message methods (#6731)

Add `email.message.Message` methods, mark `email.iterators.walk` as wontfix

stdlib/email/message.pyi

@@ -69,6 +69,9 @@ class Message:
         replace: bool = ...,
     ) -> None: ...
     def __init__(self, policy: Policy = ...) -> None: ...
+    # The following two methods are undocumented, but a source code comment states that they are public API
+    def set_raw(self, name: str, value: str) -> None: ...
+    def raw_items(self) -> Iterator[tuple[str, str]]: ...
 
 class MIMEPart(Message):
     def __init__(self, policy: Policy | None = ...) -> None: ...