webhook(): proxy and headers support

[MrAnno]

Depends on #523

webhook(port(8080) proxy-header("x-forwarded-for"));
webhook(port(8080) proxy-header(yes));

$ curl -H "x-forwarded-for: 4.3.3.2"
    -H "X-Forwarded-FOR: 1.2.3.4" -X POST --data "{}" http://127.0.0.1:8080

$PEERIP: 127.0.0.1, $SOURCEIP: 1.2.3.4

webhook(port(8080) include-request-headers(yes));

filterx {
    headers = json(${webhook.headers});

    $type = headers["Content-Type"][-1];
    $headers = headers;
};

$headers:

{
  "Host": ["127.0.0.1:8080"],
  "User-Agent": ["curl/8.5.0"],
  "X-Forwarded-For": [
    "4.3.3.2",
    "1.2.3.4"
  ],
  "Content-Length": ["2"],
  "Content-Type": ["application/x-www-form-urlencoded"]
}

$type == "application/x-www-form-urlencoded"

The merge-base changed after approval.