Consider if RDF4J Service source should be moved into CDSP 'core' tree

It was agreed with @claireqiu and @chrizmc that PR #119 would be merged and the following review finding be dealt with in a new issue (this one):
Is there a reason the RDF4J Service source is located in the Docker source tree, rather than somewhere in the CDSP core source tree in cdsp/cdsp with the other building blocks? Typically the Docker tree is reserved for Docker specific configurations files such as Dockerfiles or Docker Compose service descriptions.

Please discuss with the maintainers.

_Originally posted by @slawr in https://github.com/COVESA/cdsp/pull/119#discussion_r3640294440_

This service is still required if you were natively integrating the Playground into a linux distro without using Docker, so it appears the source should not be in the docker source tree. The question then raises where it should be placed in the CDSP core tree.

In discussion @claireqiu thinks it makes sense to keep the JAVA source for this service separate from the C++ Knowledge Layer source in the CDSP core source tree. She also explained that CQELS integration is likely bringing sw structure changes to the KL. As the RDF4J service is a black box to users it was agreed to leave it in the docker tree for now until the CQELS changes are clearer and to deal with its movement then.