MINOR: acme: get the challenges object from the Auth URL

MINOR: acme: get the challenges object from the Auth URL

This patch implements the retrieval of the challenges objects on the
authorizations URLs. The challenges object contains a token and a
challenge url that need to be called once the challenge is setup.

Each authorization URLs contain multiple challenge objects, usually one
per challenge type (HTTP-01, DNS-01, ALPN-01... We only need to keep the
one that is relevent to our configuration.