Update-with-start: shopping cart

[dandavison]

• The Workflow represents a Shopping Cart in an e-commerce application, and
  update-with-start is used to add items to the cart, receiving back the updated cart subtotal.

• Cannot be merged until UwS is released

[cretz]

Hrmm, I wonder if we should add a validator that disallows add_item if this is true. While this workflow is safe and doesn't need it, it could be a good demo, and would help users if they ever had a step between the wait condition and the return.

[cretz]

Suggested change

	from message_passing.update_with_start.lazy_initialization.activities import (
	ShoppingCartItem,
	get_price,
	)
	with workflow.unsafe.imports_passed_through():
	from message_passing.update_with_start.lazy_initialization.activities import (
	ShoppingCartItem,
	get_price,
	)

Need to encourage passing through all activity references and externally defined models instead of reloading them every workflow (for the samples this technically works because the activity doesn't do anything externally meaningful)

[yuandrew]

Can you add a little blurb as to what "lazy init" means here? Does lazy init here refer to how we're using update_with_start to initialize and start the workflow?