onAdStart
Asset assetId of brk started playing as ad index (0-based) of total.
open fun onAdStart(brk: Break, assetId: String, index: Int, total: Int, identity: AdCreativeIdentity?, asset: Asset? = null)
identity-carrying overloads for the ads of a GAM pod (PLAYG-362): IMA reports an adId/creativeId per creative that the manifest cannot know. asset is the synthetic per-ad asset when the manifest does not list it. Default to the plain callbacks so existing implementations (and fakes) that never see a pod need not override.