The implementation of this function allows Let's Encrypt to
verify that the requestor has control of the domain name.
The callback may be called once for each domain name in the
'issueCertificate' call. The callback should do whatever is
needed so that a GET on the 'url' returns the 'keyAuthorization',
(which is what the Acme protocol calls the expected response.)
Note that this function may not be called in cases where
Let's Encrypt already believes the caller has control
of the domain name.
Authorization setup callback
The implementation of this function allows Let's Encrypt to verify that the requestor has control of the domain name.
The callback may be called once for each domain name in the 'issueCertificate' call. The callback should do whatever is needed so that a GET on the 'url' returns the 'keyAuthorization', (which is what the Acme protocol calls the expected response.)
Note that this function may not be called in cases where Let's Encrypt already believes the caller has control of the domain name.