Extend ProductAvailabilityModel to include next delivery information

As a business user I would like to be able to inform my customers on the next available delivery (for example advise on next delivery time for items not in stock).

TD:

• Require additional fields on SkuWarehouse nextDeliveryDate, nextDeliveryNote
• primary usage is to use nextDeliveryDate with formatted message, secondary usage display nextDeliveryNote as is (for example for notes that come from 3rd party integrations)
• both fields should be exposed in ProductAvailabilityModel