BUG/MINOR: acme: allow "processing" in challenge requests

BUG/MINOR: acme: allow "processing" in challenge requests

Allow the "processing" status in the challenge object when requesting
to do the challenge, in addition to "pending".

According to RFC 8555 https://datatracker.ietf.org/doc/html/rfc8555/#section-7.1.6

   Challenge objects are created in the "pending" state.  They
   transition to the "processing" state when the client responds to the
   challenge (see Section 7.5.1)

However some CA could respond with a "processing" state without ever
transitioning to "pending".

Must be backported to 3.2.