vars: Allow overriding http.auth.user.id in replacer as a special case

[francislavoie]

Context: https://caddy.community/t/logging-a-response-header-field-in-caddy-reverse-proxy-without-returning-to-client/22758

I was thinking, it would probably be useful to allow users to fill http.auth.user.id for themselves using vars, because then it would show up in the access logs in the dedicated user_id field we already have.

Example config:

:8881 {
    log
    header -X-User-Id
    reverse_proxy localhost:8883 {
        handle_response {
            vars http.auth.user.id {rp.header.X-User-Id}
            copy_response
        }
    }
}

:8883 {
    header X-User-Id foobar
    respond "yo"
}

Example log, notice user_id is filled:

2024/02/15 11:30:00.866	INFO	http.log.access	handled request	{
	"request": {"remote_ip": "127.0.0.1", "remote_port": "46870", "client_ip": "127.0.0.1", "proto": "HTTP/1.1", "method": "GET", "host": "localhost:8881", "uri": "/", "headers": {"User-Agent": ["curl/7.81.0"], "Accept": ["*/*"]}},
	"bytes_read": 0,
	"user_id": "foobar",
	"duration": 0.001003354,
	"size": 2,
	"status": 200,
	"resp_headers": {"Content-Length": ["2"], "Content-Type": ["text/plain; charset=utf-8"], "Date": ["Thu, 15 Feb 2024 11:30:00 GMT"], "Server": ["Caddy", "Caddy"]}
}

[mholt]

I don't love special cases, but the benefit is compelling... let's try it :)

[mholt]

Huh, I'm having trouble getting the s390x tests to pass here. @francislavoie --- must be flaky, because they passed for the auto-merge.

[mohammed90]

Huh, I'm having trouble getting the s390x tests to pass here. @francislavoie --- must be flaky, because they passed for the auto-merge.

My fault. Will fix it in a PR.