Add support for connections via unix socket with multi-target approach

[Drugoy]

Proposal

Add support for connections via unix socket with multi-target approach

Use case. Why is this important?
Connections over unix sockets are faster / more optimal compared to network connections.
Some postgres instances in some cases might be configured to be serving only local connections over unix sockets.

p.s.: also, when this feature gets added - please add an example of its usage to the docs, because the syntax will probably be confusing.