PND related operations via NBI are located in CoreServer

Description

Currently, all the operations (add, delete, etc.) one could do on PND's are reachable through a gRPC API which is named "CoreServer". This does not seem to be fitting semantically. Previously, the PndServer was used to manage Change and NetworkElement related things, but since the refactoring of PND's the actual PndServer does not have any functionality at all. Now we have to decide if we want to keep the CoreServer for the purpose of dealing with PND related things or move the functionality of the CoreServer to the PndServer or rather just rename the CoreServer to PndServer and clean up follwing issues. Anyway, we probably we want to remove one of the servers.