This is, to my knowledge, an entirely inconsequential PR to add support for entirely novel challenge types.
Presently in the [`challb_to_achall` function](https://github.com/certbot/certbot/blob/399b932a869403b142fb0271aec510614085d6ed/certbot/certbot/_internal/auth_handler.py#L367) if the challenge type is not of a type known to certbot an error is thrown. This check is mostly pointless as an authenticator would not request a challenge unknown to it. This check does however forbid any plugins from supporting entirely novel challenges not of the key authorisation form.
* support unknown ACME challenge types
* add to changelog
* update tests
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Co-authored-by: Brad Warren <bmw@eff.org>
