Adding bill_to to supplies and costs

As clients can now assign who should be charged for individual line items for supplies and cost in our app, we have extended response in API to include those values in the response for task object.

New key bill_to has been introduced with costs and supplies arrays.

In case where values in bill_to are different, tasks bill_to will have the value multiple. Or in case this value is same across the cost and supplies, the same vallue will be populated for main bill_to in task.