[FEATURE] Add 'Addons' section to checkout

STATUS
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**READY**


Description
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Allow 'addon' items to appear on the cart page.

Purpose
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- [x] Upselling feature for 'addon' items in the cart

Deploy Notes
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Deploy as normal.
We also need to pass the `order_token` for unauthenticated/guest users... Can just add `'order_token': Spree.current_order_token` to the data for this call.