Add ability to start a http_proxy/socks5 proxy

[markpash]

Now that tnset gives us the ability to Dial and the tailscaled codebase itself already containing code to create these proxies, it should simply be a case of copying the code and making it work in the sidecar.
(We need to copy because the code that does this is unexported in cmd/tailscaled

I created a proof-of-concept to see if it works with tsnet and everything worked well with both proxy kinds.

Just need to make sure that the copied code is properly attributed, contains the copyright notices and comes with the license.