Manage activity script variables with the CLI

[nicogommen]

It seems like currently users have to use the API to manage activity scripts variables (add, delete, update). See https://docs.platform.sh/integrations/activity.html#activity-script-variables

It would be nice to be able to use platform CLI to manage these variables too.

platform variable:create
platform variable:delete
platform variable:update

Maybe with --level=activity-script or something like this.