Configure kubelet through generated configuration

[hbc (bcho)]

Summary

• map the controlled FlexNode kubelet settings (maxPods, verbosity, and image GC thresholds) into Unbounded's generated KubeletConfiguration
• expose optional kubelet exec image credential provider config and binary paths without exposing arbitrary kubelet configuration
• keep Machine goal ownership and persistence unchanged; Azure-side max pods and image GC settings are immutable, and broader authoritative goal handling remains tracked by Make ARM Machine goals authoritative and acknowledge RP node updates #256
• document credential provider rootfs/read-only mount setup and add validation, adapter, and copy coverage

Dependency

This is pinned to the current head of draft upstream PR Azure/unbounded#573. Replace the pseudo-version with the containing Unbounded release after that PR merges.

Testing

• make check