ACME Challenges are well-known strings ("http-01", "dns-01", and
"tlsalpn-01") identifying which kind of challenge should be used
to verify control of a domain. Because they are well-known and
only certain values are valid, it is better to represent them as
something more akin to an enum than as bare strings. This also
improves our ability to ensure that an AcmeChallenge is not
accidentally used as some other kind of string in a different
context. This change also brings them closer in line with the
existing core.AcmeResource and core.OCSPStatus string enums.
Fixes#5009
e2c8f6743a