update ChatCompletionResponse.Created filed ,support return Created type is string

[dragonTalon]

ChatCompletionResponse.Created filed support compatibility between int64 and string formats. and created is null

Created             FlexibleTime           `json:"created,omitempty"`

type FlexibleTime struct {
	Time int64  //openai
	Raw  string // azure
}

func (ft *FlexibleTime) UnmarshalJSON(data []byte) error {
	var timestamp int64
	if err := json.Unmarshal(data, &timestamp); err == nil {
		ft.Time = timestamp
		ft.Raw = fmt.Sprintf("%d", timestamp)
		return nil
	}
	var timeStr string
	if err := json.Unmarshal(data, &timeStr); err == nil {
		ft.Raw = timeStr
		layouts := []string{
			"2006/01/02 15:04:05",
			time.RFC3339,
			"2006-01-02 15:04:05",
		}
		for _, layout := range layouts {
			if t, err := time.Parse(layout, timeStr); err == nil {
				ft.Time = t.Unix()
				return nil
			}
		}
		return fmt.Errorf("无法解析时间字符串: %s", timeStr)
	}

	return fmt.Errorf("created 字段必须是整数或字符串")
}

Issue: #1002