The encoding of the `spacer` element looks incorrect.

The encoding of the spacer type in the flex message object appears to be incorrect. It seems necessary to add a model definition (such as in linebot/messaging_api/model_flex_spacer.go) and unmarshal logic in linebot/messaging_api/model_flex_component.go. If this approach is acceptable, I would like to proceed with submitting a pull request. Please let me know if there are any concerns or suggestions for improvement. Note: According to the FLEX MESSAGE SIMULATOR, "Spacer is no longer supported and will be removed in a future version." Are you planning to add definitions for spacer?

System Informations

Go version: go1.22.5 darwin/arm64
OS: macOS 14.4.1

Expected Behavior

{
    "type": "flex",
    "altText": "",
    "contents": {
        "type": "bubble",
        "body": {
            "layout": "vertical",
            "flex": 0,
            "type": "box",
            "contents": [
                {
                    "type": "spacer",
                    "size": "xl"
                }
            ]
        }
    }
}

Current Behavior

{
    "type": "flex",
    "altText": "",
    "contents": {
        "type": "bubble",
        "body": {
            "layout": "vertical",
            "flex": 0,
            "type": "box",
            "contents": [
                {
                    "FlexComponentInterface": null,
                    "Type": "spacer",
                    "Raw": {
                        "size": "xl",
                        "type": "spacer"
                    }
                }
            ]
        }
    }
}

Steps to Reproduce

flex := `{
    "type": "flex",
    "contents": {
        "type": "bubble",
        "body": {
            "type": "box",
            "layout": "vertical",
            "contents": [
                {
                    "type": "spacer",
                    "size": "xl"
                }
            ]
        }
    }
}`
f, _ := messaging_api.UnmarshalMessage([]byte(flex))
ff, _ := json.MarshalIndent(f, "", "    ")
fmt.Println(string(ff))

line / line-bot-sdk-go