Add StorageClass to dpa.LoadAffinity type

[kaovilai]

Add StorageClass to dpa.LoadAffinity type
velero added StorageClass to affinity so follow up to add storageclass to our types
from velero pkg/util/kube/pod.go

type LoadAffinity struct {
	// NodeSelector specifies the label selector to match nodes
	NodeSelector metav1.LabelSelector `json:"nodeSelector"`

	// StorageClass specifies the VGDPs the LoadAffinity applied to. If the StorageClass doesn't have value, it applies to all. If not, it applies to only the VGDPs that use this StorageClass.
	StorageClass string `json:"storageClass"`
}

Originally posted by @kaovilai in #1963 (comment)