Add "Offline" value for DataTransferStatus

[Pietfried]

Describe your changes

Addressing changes required by interface changes in libocpp. Added additional value for type DataTransferStatus which allows to indicate that the charging station is offline when a data_transfer command is executed. If the charging station is offline, this command will return immedieately without waiting for a response from the csms

Issue ticket number and link

Companion PR in libocpp: https://github.com/EVerest/libocpp/pull/699

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [ ] I have made corresponding changes to the documentation
• [x] I read the contribution documentation and made sure that my changes meet its requirements