Function
append-header-pending
Adds a response header with given value to a pending response.
Unlike insert-header-pending, this does not discard existing values for
the same header name, but will instead result in multiple headers of the
same name in the final response, each with their own values.
The target argument controls under which conditions this header is applied.
append-header-pending: func(pending: borrow<pending-response>, name: string, value: list<u8>, target: pending-response-kind) -> result<_, error>;