ios/android proxy: support proxy with auth

[Augustyniak]

At least on iOS, system proxy settings screen does allow it users to specify login and password that should be used to authenticate with a given proxy. We should look into whether we need to support proxy auth on Android.

^ A part of the proxy settings screen on iOS

Looking at the cronet code (networking library) - thank you @RyanTheOptimist! - it seems that we do not need to support proxy auth on Android. This is the comment that can be found in the library's code:

// - kCFProxyPasswordKey : Despite the existence of these keys in the
//                         documentation, they're never populated. Even if a
//                         username/password were to be set in the network
//                         proxy system preferences, we'd need to fetch it
//                         from the Keychain ourselves. CFProxy is such a
//                         tease.

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