Performs partial updates on existing campaigns in bulk. Only fields explicitly included
in the request body will be modified; all other campaign attributes remain unchanged.
Max batch size: 10. advertiserId must be the same across all items in the payload.
Fields can be cleared by passing null. All of budgetType, dailyBudget, totalBudget,
startDate, endDate, and deliverySpeed must be set at same level i.e. either at campaign
level or ad group level.