feature: local proxy auth

[x64BitWorm]

Problem:
Sometimes some applications (such as banking applications or others from large providers) scan local ports for open HTTP/SOCKS proxies. When such a proxy is detected, they make an unauthorized connection to it and make a request to their server through that proxy. This reveals the proxy's IP address, allowing it to be blacklisted globally on country provider level. This is currently a serious problem—the local SOCKS server is unprotected.
Article - https://habr.com/ru/articles/1020080/
POC application - https://github.com/runetfreedom/per-app-split-bypass-poc

Solution:
SOCKS server authentication has been added using login and password, which can be configured in a separate settings section.

[joky-dev]

@x64BitWorm Since you're suggesting a code fix, perhaps you have a better understanding of how this SOCKS proxy port works and why it's necessary?
I specifically tried this in this client, disabling unconditional inbound when creating this port (and removing the setHttpProxy setting in VpnService.Builder). The VPN service works fine in this mode. Are you sure we need this SOCKS5 proxy in VPN mode?

[x64BitWorm]

Sometimes there is a need to connect to this proxy via socks - for example, some browsers (like Firefox on Android) allow you to install a plugin (like https://addons.mozilla.org/en-US/firefox/addon/smartproxy/) and use custom routing rules (depending on the site) within it. Such plugins support Socks5, and sometimes you don't want to proxy the entire Firefox browser, but only specific sites via the plugin