[Core] Support PostgreSQL as SystemDB Datasource

[ThuF]

TODO:

1. Add in the issue the following environment variables in order to run SystemDB against PostgreSQL:

   DIRIGIBLE_DATABASE_SYSTEM_DIALECT: org.hibernate.dialect.PostgreSQLDialect
   DIRIGIBLE_DATABASE_SYSTEM_DRIVER: org.postgresql.Driver
   DIRIGIBLE_DATABASE_SYSTEM_URL: jdbc:postgresql://<host>:<port>/<database>
   DIRIGIBLE_DATABASE_SYSTEM_USERNAME: <username>
   DIRIGIBLE_DATABASE_SYSTEM_PASSWORD: <password>
   DIRIGIBLE_SCHEDULER_DATABASE_DELEGATE: org.quartz.impl.jdbcjobstore.PostgreSQLDelegate
   

2. Add in the documentation the following new environment variables introduced with their default values:

   DIRIGIBLE_DATABASE_SYSTEM_DIALECT: org.hibernate.dialect.H2Dialect
   DIRIGIBLE_DATABASE_SYSTEM_DDL_AUTO: update
   

Fixes: #5476