Decouple PodGroup API

This KEP proposes decoupling the PodGroup API from the Workload API by introducing PodGroup as a standalone runtime object. In alpha1 design for gang scheduling, PodGroups were embedded within the Workload spec, which creates challenges around immutability, scaling, and lifecycle management. Under this proposal, the following changes are proposed as alpha2: - Workload becomes a static template defining scheduling hierarchy and scheduling policy definition that specifies what workload behavior should be applied - PodGroup becomes a runtime self-contained runtime scheduling unit for a group of pods that encapsulates both the scheduling policy and status. It is automatically created by true controllers (Job, JobSet, LeaderWorkerSet) when they create workloads, rather than being embedded within the Workload spec. It will be created based on the podGroupTemplate defined in the referenced Workload.