Add UrlContentRequest and refactor HttpUtil
Introduce UrlContentRequest DTO to encapsulate url, timeout, httpPort, proxy credentials and userAgent. Refactor HttpUtil APIs (getUrlContent, getUrlContentWithUserAgent, downloadToFile) to accept UrlContentRequest and update callers accordingly in AngConfigManager, SpeedtestManager, UpdateCheckerManager, PerAppProxyActivity and UserAssetViewModel. This centralizes HTTP request parameters, reduces duplicated parameter lists, and preserves existing behaviors (redirect handling, userAgent fallback, proxy auth).