[Appstore] Clean code description field

[ghost]

The description field of hawkbit artifacts is currently used by the Kuksa appstore to save user-IDs. #113 mentions the description field in connection with consent management. I.e. the description field is used for a potentially growing number of aspects besides a (human-readable) description.

Following clean-code practices, it is beneficial to use dedicated fields with appropriate names for the other aspects, e.g. authorization. If hawkbit does not premit arbitrary meta-data for artifacts in the hawkbit version that we use, metadata might be available in a more recent version or we could request this feature from the hawkbit team.