The previous system was too limited when it comes to flexibility using
hooks. This limitation came from the false idea that, for a given
certificate, all challenges must be validated with the same method. In
order to prove that false, domains in a certificate can now make use of
any challenge type available.
In order to be more flexible, hooks are now given a type and are defined
in the same registry (instead of 6). Each one will be called when
considered relevant based on its type.