Add ecmr-datamodel dependency to ecmr-backend

the ecmr-datamodel dependency from the gitlab package registry has to be added as a maven dependency to the ecmr-backend.

More information aboput the dependency an the required steps can be found here in the gitlab package registry: https://git.openlogisticsfoundation.org/wg-electronictransportdocuments/ecmr/ecmr-model/-/packages/27