code generating containerd configuration need to be improved and moved to sylva-library

This issue is about 3 things for the code generating containerd configuration to sylva-library:

• the code is today duplicated in both sylva-units and sylva-capi-cluster, it would be good to move it to a single place (see discussion)
• the way validation is done to avoid is_default_mirror being set on multiple mirrors of a given registry can be improved (see sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster!656 (comment 2465571341))

/cc @daniel.anton @rletrocquer @feleouet