internal/socks: add the ability to control socks5 resolution

[sjpotter]

not all servers support remote resolution, and require one to send only ip addresses

this add a new SOCKS5 dialer creation function that lets one pass in a custom *net.Resolver.

if a resolver is set on the dialer, it will use it it convert non ip addresses into ip addresses. For instance, can simply be used with net.DefaultResolver to do local resolution

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[sjpotter]

In working more on this, I'd want to argue that there should be a Resolver interface type, so custom resolution solutions can be passed in to any place that wants a resolver. Currently, there's only a concrete Resolver type, which means any place (such as this PR) that takes a resolver cannot work as one might want.