Support containerd config_path for registry endpoints

[brandond]

Containerd has preferred using docker-style certs.d via the CRI config_path values, since the 1.5 release.
Ref: https://github.com/containerd/containerd/blob/main/docs/hosts.md#cri

[plugins."io.containerd.grpc.v1.cri".registry]
  config_path = "/etc/containerd/certs.d"

stargz should support this, instead of requiring inline config under the plugins."io.containerd.snapshotter.v1.stargz".registry.mirrors table key

Current:

[plugins."io.containerd.snapshotter.v1.stargz".registry.mirrors."docker.io"]
  endpoint = ["https://registry.example.com/v2"]

Preferred:

[plugins."io.containerd.snapshotter.v1.stargz".registry]
  config_path = "/etc/containerd/certs.d"

[brandond]

I guess this has actually been supported since #331, but I wasn't aware of that when I refactored the registry stuff in k3s-io/k3s#8973