新增导出bpmn-xml的数据格式

[Zeng-xun]

功能描述

有些后端流程框架无法解析json格式数据，希望提供方法支持json和bpmn-xml格式之间的相互转换

期望解决方案

json格式与bpmn-xml格式之间相互转换

[x6-bot]

👋 @Zeng-xun

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[The-End-Hero]

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