Provide a way to parse a single challenge

[Kladki]

Given that only a single challenge can be present per (non-WWW-Authenticate) header, would it make sense to provide a way to directly derive a single ChallengeRef from a str(ing) and/or HeaderValue?

[scottlamb]

Given that only a single challenge can be present per (non-WWW-Authenticate) header,

I don't follow. WWW-Authenticate is the usual header, or perhaps Proxy-Authenticate. What other header are you parsing that only allows one challenge?

[Kladki]

Authorization