Support manual management of transactions at the edge through a config prop

Some folk might want to use the framework without the built-in persistence mechanisms, and roll their own, eg using JOOQ or (even) event sourcing.  Or they might use JPA but prefer to minimize the length of transactions and set jpa open-in-view to false .

useful reference on this: https://www.baeldung.com/spring-open-session-in-view.

To support this, suggest we introduce a flag which if set means that the framework no longer auto-manages the transaction at the edge.  Perhaps a metamodel validation should then also enforce that there are no `@DomainObject(nature=ENTITY)` to be found in the domain model. 

Perhaps the property is called causeway.core.view-models-only? 

Or alternatively causeway.core.explicit-transaction-management?