Multiple Operator Refs in AlternativeService

[TO-mdv]

In the AlternativeServiceStructere, only one OperatorRef can be used. For the UseCase Sharing, we must have the ability to add multiple OperatorRefs. So in a TripResponse, all possible Sharing provider can be listened there.

[TO-mdv]

@ue71603 issue for multiple OperatorRefs also in the AlternativeServiceStrucutre, as discuessed yesterday

[sgrossberndt]

Thanks for the contribution.
When creating the branch please use feature/ or bugfix/ as a prefix in order to get a rendered documentation at https://vdvde.github.io/OJP/index.html for the change so you can verify looks as intended. (e.g. https://github.com/VDVde/OJP/tree/feature/trip-monitoring or https://github.com/VDVde/OJP/tree/bugfix/docu-link-missing)