Ephemeral Storage Support for Pod-Level Resource Specifications

KEP-2837: Pod Level Resource Specifications introduced the resources field at the PodSpec level (pod.spec.resources), allowing users to define aggregate compute resource requests and limits shared across all containers in a Pod. However, KEP-2837 initially scoped support strictly to CPU, memory, and hugepages-*, deferring ephemeral-storage to a future enhancement. This KEP proposes extending Pod-Level Resource Specifications to support ephemeral-storage (pod.spec.resources.requests[ephemeral-storage] and pod.spec.resources.limits[ephemeral-storage]), gated by a new dedicated feature gate: PodLevelResourcesEphemeralStorage. This aligns ephemeral storage with KEP-2837 and allows multi-container Pods to manage a unified Pod-level ephemeral storage budget across container writable layers, container logs, and disk-backed emptyDir volumes.