Adds the Create Line Item Attributes endpoint.
This extends the functionality of the Subscriptions API by adding a POST endpoint to add line item attributes to a given line item. Attributes can still be added and modified using the Update Line Items PATCH endpoint.
While the PATCH endpoint overwrites any attribute values that share the same
name for a line item, this POST endpoint instead returns an error if you attempt to send new attributes that share a
name with existing attributes.