Add a backend for OpenHarmony

The relationship between open-source OpenHarmony and HarmonyOS is similar to the relationship between AOSP and Android.

Huawei has now launched a new version, currently called HarmonyOS NEXT, which has removed AOSP and no longer supports Android applications.

> In China, Chinese documentation is updated first. It is recommended to use browser translation instead of using the language switch on the documentation.
- [OpenHarmony](https://www.openharmony.cn/mainPlay)
- [HarmonyOS NEXT](https://developer.huawei.com/consumer/cn/doc/harmonyos-releases-V5/next-dev-beta-rn-V5?catalogVersion=V5)

> [HarmonyOS API](https://developer.huawei.com/consumer/cn/doc/harmonyos-references-V5/development-intro-api-V5?catalogVersion=V5)(Kit is API)

tips:
> Statistics released by Counterpoint on September 9 show that in the first quarter of 2024, [OpenHarmony](https://gitee.com/openharmony)'s operating system accounted for 4 percent of the global market and surpassed iOS as the second largest operating system in the Chinese market.
>
> "The number of OpenHarmony ecosystem devices has exceeded 900 million as of June 2024."

### Describe the solution you'd like

Reference：
- [ohos_flutter](https://gitee.com/openharmony-sig/flutter_flutter)
- [ohos_React_Native](https://gitee.com/openharmony-sig/ohos_react_native/blob/0.72.5-ohos-5.0-release/docs/架构介绍.md)
- [ArkUI-X](https://gitee.com/arkui-x)
- [ohos_QT](https://gitee.com/openharmony-sig/qt)
- [.NET](https://github.com/OpenHarmony-NET)
- [Avalonia](https://github.com/OpenHarmony-NET/OpenHarmony.Avalonia)

### Describe alternatives you've considered

.e.g

### Additional context

_No response_

The issue names no Toga files, tests, or entry points. Start by reviewing Toga's existing backend structure and the referenced OpenHarmony projects; done would mean a defined and implemented OpenHarmony backend with project-specific validation.

Written by the indexing model from the issue text.