feat: Custom line items (#10408)

* feat: Custom line items

* fix tests

* fix migration

* Allow custom items in update line item workflow

* throw if line item doesn't have a price

* minor things

* wip

* fix flows

* fix test

* add default

* add to type

packages/core/utils/src/common/deep-flat-map.ts

@@ -68,7 +68,7 @@ export function deepFlatMap(
     const currentKey = path[0]
     const remainingPath = path.slice(1)
 
-    if (!isDefined(element[currentKey])) {
+    if (!isDefined(element?.[currentKey])) {
       callback({ ...context })
       continue
     }