Add SOCKS proxy support

[mrobinet]

This change adds support for SOCKS4 and SOCKS5 proxies. The use case I have is to test Ratpack services as if they are part of a test environment network. SOCKS proxies offer remote DNS resolution capabilities that allow the service to resolve internal hostnames for the target network/environment. I saw that Netty already has support for SOCKS proxies, but Ratpack was not exposing it.

Example usage:

HttpClient.of(spec ->
    spec.proxy(proxySpec ->
        proxySpec.host("localhost").port(7777).type(Proxy.Type.SOCKS5)
    )
);

I explored a few other options here as well. Instead of a SOCKS proxy you can just use local port forwarding, but if there are a lot of different upstreams then it can be quite tedious to forward all the ports and reconfigure the app to use the proper ports for each upstream. The JVM also has socksProxyHost and socksProxyPort flags, but unfortunately there is a bug in the JVM that prevents remote DNS from working properly when using those, so this was the next best approach.

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This change is