Overhaul the usage of libcurl-impersonate (#4535)

libcurl-impersonate was not being used properly, as the code was
overriding the headers set by it to prevent detection.

- update the libcurl-impersonate to an actively managed lexiforest
  fork
- impersonate Chrome 131
- move the defaultHttpHeaders to http.php, where it belongs
- only set defaultHttpHeaders if curl-impersonate is not detected
- make useragent ini setting optional and disabled by default
- add necessary documentation updates

lib/contents.php

@@ -64,19 +64,7 @@ function getContents(
         }
     }
 
-    // Snagged from https://github.com/lwthiker/curl-impersonate/blob/main/firefox/curl_ff102
-    $defaultHttpHeaders = [
-        'Accept' => 'text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8',
-        'Accept-Language' => 'en-US,en;q=0.5',
-        'Upgrade-Insecure-Requests' => '1',
-        'Sec-Fetch-Dest' => 'document',
-        'Sec-Fetch-Mode' => 'navigate',
-        'Sec-Fetch-Site' => 'none',
-        'Sec-Fetch-User' => '?1',
-        'TE' => 'trailers',
-    ];
-
-    $config['headers'] = array_merge($defaultHttpHeaders, $httpHeadersNormalized);
+    $config['headers'] = $httpHeadersNormalized;
 
     $maxFileSize = Configuration::getConfig('http', 'max_filesize');
     if ($maxFileSize) {