Add optional subscription-manager fact generation

[cgwalters]

We’re discussing subscription-manager integration with bootc. First step is change detection in bootc that can trigger a systemd unit.
The main debate is where the writing of /etc/rhsm/bootc.facts lives. Proposal:

• Add a cargo feature rhsm to support enabling this code (want this to cover the Debian/Arch cases)
• Add a systemd unit bootc-rhsm-facts.service triggered by our bootc-status-updated.target. This unit should have ConditionDirectoryExists=/etc/rhsm (This dynamic detection will cover the fedora case)
• The main thing to debate here is if we subpackage this as bootc-rhsm or just ship it in the main package. In a perfect world we’d subpackage it. But there are logistical overheads to doing this. Decision: Not a subpackage.