Add a bootc centos stream 9 host

[ianballou]

I built a base CentOS 9 Stream bootc host and uploaded it as a box to https://portal.cloud.hashicorp.com/services/vagrant/registries/katello/boxes/centos-stream9-bootc/versions/2025.0925.1700?project_id=08e72f3b-c08f-4707-a94b-96f7464fbf9a

It's exposed here in forklift. There are some caveats:

1. ~~I had to find the raw download link because Fedora's current version of Vagrant doesn't seem to know how to connect to the new Vagrant registry in the Hashicorp cloud (https://app.daily.dev/posts/vagrant-cloud-is-moving-to-hcp-s6pzswzuc)~~
2. Bringing the machine up fails with mkdir: cannot create directory ‘/vagrant’: Read-only file system because / is read-only on bootc hosts. -> I tried disabling the syncing via synced_folders in the yaml file, but it was ignored. I'm curious if anyone has any other ideas for disabling the default rsyncing for a single host.