make clean_error_msg public for protocol class users

[Coservit]

Hello,
first, thanks for this library, really useful when managing windows host.
It seem the winrm.session class does not handle other code page except ascii, and the recommended way to go is using the protocol object if we need other codepage (see issue 258, 357, 333)
I use the protocol object successfully, but need to decode the clixml message if there is an error.
for that _clean_error_message is a useful method, but it's not accessible from the protocol object, and it's not public.
could you make this function public for everyone to use, without having to create a session object ?
tested on python3.10