Add image command to show supported rootfs/kernel images

[innobead]

Is your feature request related to a problem? Please describe.
Having a command to show supported images instead of documentation.

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[innobead]

➜  kubefire (master) ✔ ./target/kubefire image         
IMAGE                                                   TYPE   
ghcr.io/innobead/kubefire-centos:8                      RootFS  
ghcr.io/innobead/kubefire-ubuntu:18.04                  RootFS  
ghcr.io/innobead/kubefire-ubuntu:20.04                  RootFS  
ghcr.io/innobead/kubefire-ubuntu:20.10                  RootFS  
ghcr.io/innobead/kubefire-opensuse-leap:15.1            RootFS  
ghcr.io/innobead/kubefire-opensuse-leap:15.2            RootFS  
ghcr.io/innobead/kubefire-ignite-kernel:4.19.125-amd64  Kernel  
ghcr.io/innobead/kubefire-ignite-kernel:5.4.43-amd64    Kernel