Trust user installed certificates

[johnstef99]

Instead of trusting all certificates using

client.badCertificateCallback =
            (X509Certificate cert, String host, int port) => true;

I created a plugin flutter_user_certificates_android for android that retrieves the CA certificates installed on the device by the user, so the HttpProxy override can trusts them.

On iOS this works out of the box with flutter and a request signed by a certificate installed by the user doesn't throw an Exception.

So by removing the badCertificateCallback:

• iOS will automatically allow any request signed by a user installed certificate
• Android will get the user certificates from the device and set them as trusted on any HttpClient created

This makes the plugin safe for use in production environments.