anaconda: Add experimental anaconda-based installation

[cgwalters]

Implement bootc container installation using anaconda as the installation
engine with kickstart processing. Uses the ephemeral VM infrastructure to
run anaconda in an isolated environment with proper access to block devices
and container storage.

Key implementation details:

• User must provide kickstart file with partitioning and locale settings
• bcvk injects ostreecontainer directive with --transport=containers-storage
• Inject %pre script to configure container storage with host overlay
• Inject %post script running 'bootc switch --mutate-in-place' to repoint
  the installed system to the registry image (for bootc upgrade to work)
• Handle SSH exit code 255 as success when VM powers off after installation
• Share disk creation logic via qemu_img::create_disk()

Options:

• --kickstart (-k): Required kickstart file path
• --target-imgref: Registry image for bootc origin (defaults to image arg)
• --no-repoint: Skip %post repointing if user handles it themselves

The anaconda installer container (localhost/anaconda-bootc) is based on
fedora-bootc with anaconda-tui installed. Build instructions in
containers/anaconda-bootc/.

[gemini-code-assist]

Code Review

This pull request introduces an experimental anaconda-based installation method for bootc images, including a new Dockerfile, documentation, and integration tests. While the code is well-structured, there are significant security concerns regarding how user-supplied image names and references are handled when generating Kickstart files. Specifically, the code is vulnerable to both Command Injection in the %post script and Kickstart Directive Injection via the image argument. These should be addressed by properly quoting and validating all user-supplied strings. Additionally, a minor issue was noted regarding a hardcoded value in the generated kickstart script, which could be made dynamic for better flexibility.