Add getShippingAddress() to order interface (resolves #6919)

[schmengler]

Description

Adding getShippingMethod() to the order interface, analogous to the existing getBillingMethod().

Fixed Issues (if relevant)

1. API improvement: OrderInterface should have a getShippingAddress method #6919: API improvement: OrderInterface should have a getShippingAddress method API improvement: OrderInterface should have a getShippingAddress method #6919

Manual testing scenarios

1. GET an order via API
2. See if there is a "shipping_address"

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[Vinai]

Since it is a data object, each getter should have a matching setter.

[adragus-inviqa]

@Vinai - Dunno. I wouldn't want setters in my interfaces. They invite mutability. Let the implementation figure it out, not the interface. You shouldn't really know or care that the implementation is a data object anyway.

[schmengler]

For consistency, I added the setter as well

[ishakhsuvarov]

@schmengler Unfortunately, due to backward compatibility policy we can not accept this Pull Request now, as it modifies an interface.
Please use ShippingAssignment extension attribute to process shippingAddress.
Thanks.