ios/android proxy: support proxy with auth

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.
```
[source](https://source.chromium.org/chromium/chromium/src/+/main:net/proxy_resolution/proxy_resolver_mac.cc;l=332?q=net%2Fproxy_resolution%2Fproxy_resolver_mac.cc&ss=chromium)