Allow customizable behavior on cors failure

[ablankz]

When CORS fails, the current process always takes the same behavior as shown below, which is not applicable to a wide range of use cases.

Therefore, it would be easier to use if the following functions could be optionally set.

// ErrorHandler is a custom error handler function for handling CORS errors.
// if you want to write success response or call next handler, return true.
// If you want to terminate further processing, return false.
ErrorHandler func(w http.ResponseWriter, r *http.Request, cors Cors, err error) bool

We also suggest that you define your own error structure to identify the errors passed here.

// Error is an interface for CORS errors.
type Error interface {
	CorsError()
}

// PreflightError is an interface for preflight errors.
type PreflightError interface {
	PreflightCorsError()
}

// ActualRequestError is an interface for actual request errors.
type ActualRequestError interface {
	ActualRequestCorsError()
}

// PreflightNotOptionMethodError is returned when the method is not allowed.
type PreflightNotOptionMethodError struct {
	Method string
}

func (e *PreflightNotOptionMethodError) Error() string {
	return fmt.Sprintf("Preflight aborted: %s!=OPTIONS", e.Method)
}

// Is implements the Is method of the error interface.
func (e *PreflightNotOptionMethodError) Is(target error) bool {
	_, ok := target.(*PreflightNotOptionMethodError)
	return ok
}

// As implements the As method of the error interface.
func (e *PreflightNotOptionMethodError) As(target any) bool {
	switch target.(type) {
	case **PreflightNotOptionMethodError:
		return true
	default:
		return false
	}
}

// PreflightCorsError implements the PreflightCorsError interface.
func (e *PreflightNotOptionMethodError) PreflightCorsError() {}

// CorsError implements the CorsError interface.
func (e *PreflightNotOptionMethodError) CorsError() {}

The above code is just an example, and we believe that the structure should be separated for each error that is currently possible as a failure.