gh-127794: Validate email header names according to RFC 5322 (#127820)

`email.message.Message` objects now validate header names specified via `__setitem__`
or `add_header` according to RFC 5322, §2.2 [1].

In particular, callers should expect a ValueError to be raised for invalid header names.

[1]: https://datatracker.ietf.org/doc/html/rfc5322#section-2.2

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: R. David Murray <rdmurray@bitdance.com>