Add fetching product activations from SCC

[felixsch]

card: https://trello.com/c/hnFo5GtQ/3724-rr2-library-add-systems-activations-functionality

This adds fetching activation from /connect/systems/activations and enables library users to determine which products are already activated on their system, as well as have a minimal input into the subscription consumed.

How to test:

$ cd <connect>
$ git checkout library-support-fetching-activations
$ make build
$ env REGCODE=<you know it!> ./out/public-api-demo SLES 15.6 x86_64
# expect: There is step 6 which shows all activation of the SLES system

As always, if you need help testing this merge request or anything else, do not hesitate to reach out to me!🚀

Thank you!