Expansion path missing for custom field type

[katmatt]

This was reported by an user via Jira: They try to create an expansion path for cart line item prices: lineItems[*].price.custom.type.id.

This currently isn't supported by our PriceExpansionModel. A quick check showed that custom fields are missing for all of our expansion model. So fixing this issue will require some more work. For now I recommend to use the ExpansionPath.of(String) method to create such an expansion path.