Handling deferred attributes automatically

Right now if an entity has a navigation property, the object must be completely reloaded (as below) to load in deferred data in navigation properties. For example:

orders = service.orders.get_entities()

for o in orders:
    do_some_stuff_with(o.Customers)  # This is an empty list right now so nothing will get done.

# The data must be reloaded as per the below for anything to happen.
orders = service.orders.get_entities().expand("Customers")

I propose that we use the OData __deferred attribute to fire a request on demand to load these attributes in. This would allow the API to look like this:

orders = service.orders.get_entities()

for o in orders:
   do_some_stuff_with(o.Customers)  # Request is automatically made using the `__deferred` attribute which transparently offers the missing data back to the class.

WDYT: @filak-sap

Start by tracing service.orders.get_entities(), navigation properties such as Customers, and the existing expand("Customers") behavior. Determine how the OData __deferred attribute could trigger on-demand loading while preserving the entity API; done means accessing o.Customers transparently provides the deferred data without explicitly calling expand.