Add support for UNIX sockets

[pinaraf]

Is your feature request related to a problem? Please describe.
PgCat, unlike Pgbouncer, doesn't support UNIX sockets, neither to connect to servers nor to expose itself to applications.
UNIX sockets allow for a slightly better latency, and allow the use of file-level authentication.

Describe the solution you'd like
PgCat should be able to connect to PostgreSQL instances using UNIX sockets.
PgCat should be reachable through UNIX sockets.

Describe alternatives you've considered
None

Additional context
None

[levkk]

This is certainly good to have, but we won't be using this in production because PgCat, clients, and Postgres will always be on separate servers. Still, good feature to have for smaller deployments on the same machine.

[shikanime]

I have a rough idea of how to add this feature, I may look into it in the near future.