OZG-2220 grpc for retrieving all supported Organisationseinheit IDs

TODO

...interface/src/main/proto/organisationseinheit.model.proto

...nager-interface/src/main/proto/organisationseinheit.proto

...erver/src/main/java/de/itvsh/kop/user/UserRepository.java

...r-server/src/main/java/de/itvsh/kop/user/UserService.java

...organisationseinheit/OrganisationsEinheitGrpcService.java

...src/test/java/de/itvsh/kop/user/UserRepositoryITCase.java

...r/src/test/java/de/itvsh/kop/user/UserRepositoryTest.java

...rver/src/test/java/de/itvsh/kop/user/UserServiceTest.java

...nisationseinheit/OrganisationseinheitGrpcServiceTest.java