Future proof the parsing of challenge types

[kellpossible]

When testing this with pebble https://github.com/letsencrypt/pebble this library fails to parse some of the challenge types that it offers. It would be good to make this enum #[non_exhaustive] and perhaps have an option for parsing the list of challenges to either ignore unknown types, or alternatively we could add a new variant something like:

#[derive(Clone, Debug, Deserialize, Eq, PartialEq)]
#[allow(missing_docs)]
#[non_exhaustive] // It's likely more variants may be added in the future.
pub enum ChallengeType {
    #[serde(rename = "http-01")]
    Http01,
    #[serde(rename = "dns-01")]
    Dns01,
    #[serde(rename = "tls-alpn-01")]
    TlsAlpn01,
    Unknown(String),
}

[djc]

Long-term API stability hasn't really been an important goal so far (so I'm not sure #[non_exhaustive]'s costs would outweigh its benefits at this stage), but I'd be open to adding an Unknown variant for sure.