Adds sk_lookup socket steering example

[murrayh]

If you haven't noticed already, there is a programming challenge going on and it looks like a few of us decided to tackle it with redbpf. Amazing technology, thank-you.

You might want to leave this for a few weeks and see whatever other sk_lookup PRs you get. You can pick the best one, or combine them or something. For example, I really like the more rustic bindings in https://github.com/foniod/redbpf/pull/228.