Add proxy support for iOS

[yerkejs]

Connection with issue(s)

Connected to #2244

Testing and Review Notes

1. Connecting proxy (HTTP or SOCKS) like Charles, Proxyman and etc

To Do

• double check the original issue to confirm it is fully satisfied
• add testing notes and screenshots in PR description to help guide reviewers

[pichillilorenzo]

This is cool, thanks! However, the API should not contain any "android" or "ios" prefix as this is something that was removed when coming from plugin version 5.
This is because you never know what setting or method you can implement and for what platform.
In general, I simply add it to the docs of the field/method/class if it is supported or not by a specific platform.

In this case, probably we could update the implementation of types/proxy_rule.dart with the iOS/macOS specific-fields, such as "bool allowFailover;" and in the code docs you would have something like this:

  /// A Boolean that indicates whether or not a proxy configuration allows failover to non-proxied connections.
  /// Failover isn’t allowed by default.
  ///
  ///**Officially Supported Platforms/Implementations**:
  ///- iOS
  bool allowFailover;

Because WebKit.WKWebsiteDataStore.proxyConfigurations accepts an array of ProxyConfiguration, this way we can set an array of different configuration.

What do you think?

[yerkejs]

Thanks for feedback, yeah makes sense, I'll update PR