Introduce value keys where the answer types can not be inferred from labels
Update the Metadata in conjunction with the new ability to have value keys in answer options.
These are used if a value of the answer type can not be inferred from a label. See the MR in Surveyval: https://gitlab.hzdr.de/hifis/overall/surveys/hifis-surveyval/-/merge_requests/147
Note that earlier Surveyval versions will likely not tolerate this change.