feat(Webdav): Add overwrite support for moveFiles and copyFiles

[xkeyC]

完善 Webdav 的 overwrite 操作，即 overwrite 为 true 时，先删除目标文件再进行后续操作。

我正在使用 Swift backup 备份手机数据，该软件具有 webdav 支持。

但我在更新备份时，会报告 Server 500 错误，问题发生在 webdav 的 moveFiles 操作。
根据我的分析，确认是 overwrite 未实现带来的文件名冲突问题。
此 PR 目的为修复这个错误。

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[HFO4]

Thanks!