feat: Adding currency field to top level of Order (#210)

* Adding currency field to top level of Order
* Improving the currency description in Order
* Update currency field description in source/schemas/shopping/order.json

Co-authored-by: Ilya Grigorik <ilya@grigorik.com>

Author: deinck6

source/schemas/shopping/order.json

@@ -82,6 +82,10 @@
       },
       "description": "Append-only event log of money movements (refunds, returns, credits, disputes, cancellations, etc.) that exist independently of fulfillment."
     },
+    "currency": {
+      "type": "string",
+      "description": "ISO 4217 currency code. MUST match the currency from the originating checkout session."
+    },
     "totals": {
       "type": "array",
       "items": {