RFC 8555 § 7.3.5 is not listed as optional but we do not currently
support it. Rather than 404, return a 501 to inform clients that this
functionality is not yet implemented.
The notImplmented error type is not an official error registered in the
ietf:params:acme:error namespace, so prefix if with step:acme:error. An
ACME server is allowed to return other errors and clients should display
the message detail to users.
Fixes: https://github.com/smallstep/certificates/issues/209
When constructing links we want to read the required host and protocol
information in a dynamic manner from the request for constructing ACME
links such as the directory information. This way, if the server is
running behind a proxy, and we don't know what the exposed URL should
be at runtime, we can construct the required information from the
host, tls and X-Forwarded-Proto fields in the HTTP request.
Inspired by the LetsEncrypt Boulder project (web/relative.go).
