Introduce abstract datachannels in Root Canal.

This introduces a set of interfaces that abstracts the network layer.
This makes it possible to use different communications channels
(gRPC/qemu pipe) and allows us to easily port root canal to other
platforms (windows/macos).

The following interfaces are introduced:

- AsyncDataChannel: A thing that can send/receive bytes.
- AsyncDataChannelServer: A thing that can accept incoming AsyncDataChannels.
- AsyncDataChannelConnector: A thing that can make outgoing connection

There is an implementation for Posix sockets, that can be used on both
Linux & Darwin (+M1).

Test: Unit tests for basic posix sockets.
Bug: 186567864
Change-Id: Ie1c2ac05abaaec691985565ed523bc65e04305eb