KEP-127 (UserNS): allow customizing subids length

The number of subuids and subgids for each of pods is hard-coded to 65536,
regardless to the total ID count specified in `/etc/subuid` and `/etc/subgid`:
https://github.com/kubernetes/kubernetes/blob/v1.32.0/pkg/kubelet/userns/userns_manager.go#L211-L228

This is not enough for some images.
Nested containerization needs a huge number of subids too.

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

keps/sig-node/127-user-namespaces/README.md

@@ -335,6 +335,8 @@ bool `pod.spec.hostUsers`.
 The mapping length will be 65536, mapping the range 0-65535 to the pod. This wide
 range makes sure most workloads will work fine. Additionally, we don't need to
 worry about fragmentation of IDs, as all pods will use the same length.
+The mapping length (multiple of 65536) will be customizable via a new
+`KubeletConfiguration` property `subidsPerPod`.
 
 The mapping will be chosen by the kubelet, using a simple algorithm to give
 different pods in this category ("without" volumes) a non-overlapping mapping.