feat: add-storageuri-targetjob-validation

[pulkit-999]

• Add regex pattern validation for StorageUri fields in DatasetInitializer
• Add name format validation for PodTemplateOverrideTargetJob.Name to match Kubernetes naming conventions

[google-oss-prow]

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[Copilot]

Pull request overview

This PR tightens API-level validation on the trainer.kubeflow.org/v1alpha1 TrainJob types by adding regex constraints for dataset storageUri values and for PodTemplateOverrideTargetJob.Name to better align with Kubernetes-style naming.

Changes:

• Add a kubebuilder Pattern validation for DatasetInitializer.StorageUri to restrict values to either scheme://... or absolute paths.
• Add a kubebuilder Pattern validation for PodTemplateOverrideTargetJob.Name to enforce DNS-1123 label-like formatting.