Fix EZP-22522: Unable to install eZ Publish 5.3 on Postgres

This PR resolves issue https://jira.ez.no/browse/EZP-22522

We handle Postgres and SQLite specifics in subclasses of ConnectionHandler. That means for these DBMS handler can't be created through constructor of main ConnectionHandler.

The problem is fixed by adding and utilizing new ConnectionHandler static method `createFromConnection`, similar to already existing method `createFromDSN`.