Support PostgreSQL schema other than public

CakePHP 4 database configuration respects the PostgreSQL notion of a schema (aka namespace) within a logical database other than the default schema named "public".

DBAL appears not to respect it with configuration, but it is straightforward to have the DBAL connection object run a command that sets the PostgreSQL search path to the schema that is configured in the CakePHP 4 database configuration.

Not using the "public" schema by default is considered a good security practice, especially when using a logical database that may have read-only users or other users with less privileges.