startGamSession
abstract suspend fun startGamSession(identity: GamSessionIdentity, assetParameters: Map<String, String>?)
Initialize the GAM/IMA stream session for identity; suspends until ready.
identity comes from the break manifest's root vendorConfiguration, so this can only be called once the manifest has been fetched.
assetParameters is the SESSION layer only — the manifest's per-asset layer is applied later by the renderer, since which asset will play is not knowable here.