Changes to Create a fixed charge

added subschema #2 to the fixed_charge request property allOf list

the response property fixed_charge/invoice_display_name became nullable for the status 200

the fixed_charge/units response's property type changed from number to string for status 200

added the optional property fixed_charge/taxes/items/applied_to_billing_entities_codes to the response with the 200 status

the response property fixed_charge/taxes/items/description became required for the status 200

endpoint added