cmd: add new describe-image command

[mvo5]

[draft as it is build on top of #6]
[edit: this should follow https://github.com/osbuild/images/pull/1104, i.e. this output should be used as input in https://github.com/osbuild/images/pull/1104]

This commit adds a new describe-image comamnd that contains
the details about the given image type. The output is yaml as
it is both nicely human readable and also machine readable.

The output looks like this:

$ ./image-builder describe-image rhel-9.1 tar
distro: rhel-9.1
type: tar
arch: x86_64
os_vesion: "9.1"
bootmode: none
partition_type: ""
default_filename: root.tar.xz
packages:
  include:
    - policycoreutils
    - selinux-policy-targeted
    - selinux-policy-targeted
  exclude:
    - rng-tools

Thanks to Ondrej Budai for the idea and the example.