Add internal property to ACME challenge error response logs

[hslatman]

This PR adds the internal error message to the ACME response log. This provides more information in the event of an ACME challenge failing to be completed, such as network connectivity issues.

Technically this isn't the correct thing to do, as the internal error message isn't part of the response that's sent to the ACME client by the CA. Furthermore, the error message will be logged on INFO level, whereas it might be more fitting to log it on ERROR. Despite that, doing it like this keeps the internal error close to the corresponding ACME request, so it seemed like a pragmatic solution to me.

The logging logic has only been changed for the Challenge type, but can be easily extended to other ACME types if this approach makes sense.

A log line will now look something like this (note the response object):

INFO[0039] duration=30.003247083s duration-ns=30003247083 fields.time="2025-03-27T11:05:44+01:00" 
method=POST name=ca nonce=UW80Q2hWRDJTdkZOa0dLU3BzYVlIUkFtQ1R2QnZqWlM path=/acme/acme/challenge/cdz67FVppDAFoLEQihPhQGwhlIeQ6obU/iTlKgajNu9ObGHHEHZ0bujAKlgYUcOG7 
protocol=HTTP/2.0 referer= remote-address=127.0.0.1 request-id=7a227ad4-1495-45c3-af73-f80c7deb75db 
response="{\"type\":\"http-01\",\"status\":\"pending\",\"token\":\"XQgTblLaYhkNZjMSltYUx35co0eAsFFI\",\"url\":\"https://127.0.0.1:8443/acme/acme/challenge/cdz67FVppDAFoLEQihPhQGwhlIeQ6obU/iTlKgajNu9ObGHHEHZ0bujAKlgYUcOG7\",\"error\":{\"detail\":\"The server could not connect to validation target\",\"internal\":\"error doing http GET for url http://10.0.0.1/.well-known/acme-challenge/XQgTblLaYhkNZjMSltYUx35co0eAsFFI: Get \\\"http://10.0.0.1/.well-known/acme-challenge/XQgTblLaYhkNZjMSltYUx35co0eAsFFI\\\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)\",\"type\":\"urn:ietf:params:acme:error:connection\"}}" 
size=316 status=200 user-agent="Smallstep CLI/0000000-dev (darwin/arm64)" user-id=

A slightly different solution could be to handle the internal error message separately from the response near https://github.com/smallstep/certificates/blob/master/api/log/log.go#L51-L99.