adding PAC based proxies on iOS

[aleufms]

• adding PAC-based proxies on iOS through WPAD (Web Proxy Autodiscovery Protocol). So, the network inspector on BrowserStack will work.
• adding support to check all proxies and get the right one instead of try to use the first one. On my system, fox example, I have the Auto proxy discovery proxy from my company always on, and it not work with http proxies in my machine (like Charles proxy or Proxyman), because the first proxy is always is the proxy of my company.
• I based the code from https://github.com/BrowserStackCE/flutter_system_proxy

[baldarama]

will this changes add support for BrowserStack Local testing feature?

im very interested in supporting that on flutter to be able to test services running locally or in internal network