Add Unix socket support to PostgreSQLPersistentStore

[craiglabenz]

Google's serverless Cloud Run framework is a great option for deploying Aqueduct applications, but it requires Unix sockets when connecting to Cloud SQL.

Luckily, as of version 2.2.0, Postgres supports Unix sockets.

So to close the loop on all of this, it would be great if Aqueduct's PostgreSQLPersistentStore constructors exposed this new isUnixSocket behavior.

[vixez]

I would need this as well, hopefully this gets implemented soon.