Add support for different api versions

- vendor.conf now has a default api version:
    - apiVersion = 3 is set for all current providers
- provider.py (called during `make vendor`) now supports api version
  and writes it to providers.json
- the desktop client logs the used api version. Currently v5 is not yet implement.
  This is just the preparation for it
- this is only a temporary solution. In the future, we want to get the
  api version during runtime by parsing $api/provider.json