[3/4] public library: Implement product activation and extension traversal

[felixsch]

related card: https://trello.com/c/uRZTaz4g/3682-rr1-library-implement-announce-activate-de-activate-and-de-register

This is the public library implementation which is required to allow Rancher Manager to register against SCC.
The general idea is to easily allow products to register themself to SCC and therefore show up as activated product in the systems details.

In this merge request:

• Add product activation/deactivation
• Add product tree traversal helpers

About the pull requests

There are multiple merge requests which all together implement a useful subset of the overall API functionality of SCC.
[1/4] public library: Implement HTTP connection
[2/4] public library: Implement registration and status/keepalive
[3/4] public library: Implement product activation and extension traversal
[4/4] public library: repository cleanup

I you are here to review these, please start with the first one (this one) and only start with the next merge request when this one is merged. I will update bases accordingly then. Each merge request enhances the public-api-test binary to show demo usage of the implemented functionality in this merge request.

If you want to see a full CI run of this set checkout: #281

How to review this merge request (and all others):

• Checkout the branch and make sure the documentation looks sane. You can render the documentation using:

$ go install golang.org/x/pkgsite/cmd/pkgsite@latest
$ pkgsite -open .

• Run the public-api-test application including a demo showing the implementation working

$ make build
$ REGCODE=<regcode> ./out/public-api-test SLES 15.5 x86_64

• Check if you can spot inconsistencies regarding the implementation. Any areas of improvements?

If you have any questions, need help setting up development environment, do not hesitate to reach out to me 🚀

Thank you! ❤️