Add an example with Spring Data JPA

[vittorioexp]

• The current example is moved inside the "01-dgs-framework-basic-features" folder.
• Another folder is created, "02-dgs-framework-with-spring-data-jpa", containing an example application for the DGS Framework which is standalone GraphQL server in Java that interacts with a PostgreSQL database via Spring Data JPA.
• The README.md files have been modified accordingly.