Support Unix-domain socket connection

[cyrilgdn]

Discussed in https://github.com/cyrilgdn/terraform-provider-postgresql/discussions/113

^{Originally posted by hasantayyar August 11, 2021} I see the the provider converts all host and port inputs to postgresq://... connection string format.

Is it possible to establish connection via sockets?

The go library requires the socket directory and the filename as hostname without postgres:// schema.