Support unix sockets

[aviramha]

Support making outgoing connections from to unix sockets on the remote pod.

Requested by customer who's application reads from the containerd socket.

[aviramha]

blocked by rust-lang/socket2#332

[aviramha]

Adding more information, as I initially started implementing it and have some thoughts:

1. This should be part of the "outgoing" configuration, a field named unix (feel free to suggest other) that accepts path patterns to connect remotely.
2. This should leverage the TCP outgoing handler, as it is pretty similar to it in terms of everything (connect, send/recv).
3. Above point is debatable, but I don't really think that splitting and complicating the code makes sense.

[t4lz]

2. This should leverage the TCP outgoing handler

Does this mean that for now we are only going to support TCP over unix sockets, not UDP?

[aviramha]

It's possible to have UDP over unix sockets? For some reason I didn't think it's possible.
I suggest we start with TCP over unix then have another issue for UDP (to be prioritized when needed).