Draft: slapos_subscription_request: create Subscription Request from Software/Slave Instance

This is another implementation of https://lab.nexedi.com/nexedi/slapos.core/-/merge_requests/789

Instead of copying the Open Sale Order logic, this MR directly uses Open Sale Order.

There is 1 invoice line per Software/Slave instance, making the monthly invoice more difficult to read.

This is only a prototype for now.