Enhanced postgres persistence configuration - avoid public schema as default

### Is your feature request related to a problem? Please describe.

Hello,

Polaris tests of deployment with eclipseLink and Postgres : I would like to be able to declare the name of the schema in the persistence.xml config file.

- Today the tables are stored in the 'public schema' of configured database.

- At least, if not configurable from persistence.xml, please set default schema as something related to Polaris, but really not public.

### Describe the solution you'd like

Not storing Polaris tables in public schema of Postgres when using eclipselink.

### Describe alternatives you've considered

- hardcoded default schema related to Polaris
- configurable schema from persistence.xml

### Additional context

Tested with eclipseLink getting started and Polaris v1-incubating

Start with the persistence.xml configuration used by the EclipseLink getting-started setup and trace how Polaris creates its PostgreSQL tables. Determine whether the schema can be configured there or whether a Polaris-specific default is appropriate. Done means Polaris tables no longer use PostgreSQL's public schema and the behavior is covered for the EclipseLink setup.

Written by the indexing model from the issue text.