Add module for product migrations

It would be good to have a module that schedules product migrations, e.g. from SLES 15 SP6 to SP7.

Needed API call is: [`scheduleProductMigration`](https://www.uyuni-project.org/uyuni-docs-api/uyuni/api/system.html#apidoc-system-scheduleProductMigration-loggedInUser-sid-baseChannelLabel-optionalChildChannels-dryRun-earliestOccurrence).