[#64043152] Use core gem

Unit tests pass, integration tests passes and it seems to work. However, this needs testing and reviewing very carefully because we don't want to break anything. Ideally this should have no impact on the users at all.

Does it still work for the way IDA are working with the tools? Also to check there are no files that are now both in this repository and https://github.com/alphagov/vcloud-core (if so, they should be removed from here).