Implements cli connection via local unix socket as described in #148:

We also need to implement a unix socket cluster connector for this to work. Should look similar to the TCP one.

Supports connection configs:

current_context: local
contexts:
  local:
    connections:
      - unix: /run/uncloud/uncloud.sock

And running cli with connect option:

uc --connect unix:///run/uncloud/uncloud.sock ...

Tested on my cluster.