fix: Add TPROXY support for client-MITM connections

[alxbl]

This PR implements a clean version of the patch documented in #204. It also splits the transparent proxying configuration examples to a different file to keep the main README clean.

I'd like @obilodeau to merge his documentation from #204 in docs/transparent-proxy.md and then we can close that issue as well.

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Closes #204, Closes #64

[obilodeau]

TProxy changes are great! Otherwise, it is mostly little things here and there to cleanup.

The bulk of the comments highlight the fact that we should merge L2 and network namespace approaches as one.

I proposed @alxbl to do it. I'll take it from here and ask him to review once I'm done.

[alxbl]

I assume this means you'll be taking over the PR and committing those changes you mentioned. I'll let you take care of it, otherwise let me know.

[obilodeau]

I re-tested everything in my environment. L2, L3 w/ option 1 and option 2.

I was able to remove the requirement for a local IP in option 1. So it's either: you do ARP (option 1) or you pin ARP (option 2). I validated by looking at a tcpdump of ARP traffic on the MITM host as I was intercepting RDP.

@alxbl I let you do the final ACK/merge.

[alxbl]

LGTM!