Today we have `kernel-rt` in extensions...but conceptually it's not an extension - it actually replaces the base (throughput) kernel.

For users who want to apply a hotfix for kernel-rt things will get confused here if they applied `kernelType: realtime` via MachineConfig because they'll actually have to override `rhel-coreos-8-extensions` and we don't have a knob for that in the MCO today.

I think what would make everything much nicer here is:

• Change rhcos build process to produce a new `rhel-coreos-8-rt` container image
• Ship it (by referencing in MCO)
• Change the implementation of `kernelType: realtime` in the MCO to imply choosing the `rhel-coreos-8-rt` base image instead of installing RPMs