[Suggestion] Allow setting app label in assemble manifest file

**Is your feature request related to a problem? Please describe.**
There is an `exported_bins_path` flag for binaries but no way to declaratively change the exported apps label.

**Describe the solution you'd like**
I would like something like `exported_apps_label` that maps to `distrobox-export --export-label`

**Describe alternatives you've considered**
I tried writing a bash script to export a list of apps as an init hook but `distrobox-export` can't run as root

查看 assemble manifest 檔案格式以及現有的 `exported_bins_path` 旗標。在程式碼庫中找出呼叫 `distrobox-export` 的位置，可能是在處理應用程式匯出的指令碼中。新增一個 `exported_apps_label` 旗標，將 `--export-label` 傳遞給匯出指令。修改一個 assemble manifest 並確認標籤已套用，以進行測試。

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