Added support for authenticated proxies to Armeria backend

[threeseed]

No description provided.

[adamw]

Ah, asJavaProxySelector indeed doesn't take into account authentication. Do you think it's possible to support onlyProxyHosts / nonProxyHosts in your code? Also, is it correct to always add proxy config, even if none is specified in the backend options?

[threeseed]

a) It works with your existing asJavaProxySelector code which already handles onlyProxyHosts/nonProxyHosts.

b) ProxyConfig.DIRECT means no proxy as described here so should be fine.

c) Also for reference I tried other approaches i.e. Java.net.Authenticator and a decorator that adds the Auth headers but neither worked.

[adamw]

Ok, fair enough, let's merge this then. Thanks! :)