feat(runtime): add getPluginInstance API

[Aarebecca]

• 新增 getPluginInstance API，用于获取插件实例并提供插件 API 调用

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• Added getPluginInstance API to get plug-in instances and provide plug-in API calls

const graph = new Graph({
  // ...
 plugin: [{key: 'history', type: 'history'}]
});

const history = graph.getPluginInstance<History>('history');

history.undo();