feat(config): add ability to customize HTTP status codes Anubis returns (#393)

Signed-off-by: Xe Iaso <me@xeiaso.net>

test/unix-socket-xff/test.mjs

@@ -0,0 +1,30 @@
+async function testWithUserAgent(userAgent) {
+  const statusCode =
+    await fetch("https://relayd.local.cetacean.club:3004/reqmeta", {
+      headers: {
+        "User-Agent": userAgent,
+      }
+    })
+      .then(resp => resp.status);
+  return statusCode;
+}
+
+const codes = {
+  allow: await testWithUserAgent("ALLOW"),
+  challenge: await testWithUserAgent("CHALLENGE"),
+  deny: await testWithUserAgent("DENY")
+}
+
+const expected = {
+  allow: 200,
+  challenge: 401,
+  deny: 403,
+};
+
+console.log("ALLOW:    ", codes.allow);
+console.log("CHALLENGE:", codes.challenge);
+console.log("DENY:     ", codes.deny);
+
+if (JSON.stringify(codes) !== JSON.stringify(expected)) {
+  throw new Error(`wanted ${JSON.stringify(expected)}, got: ${JSON.stringify(codes)}`);
+}