Add support for pausing and editing a network request before sending

In the DevTools survey, this was reported in the section: "Compared to other developer tools that you are familiar with, what are the features you think are missing in Flutter DevTools?"

该 issue 仅包含一份 DevTools 调查回复，没有指明任何文件、测试或入口点。首先需要梳理现有的网络请求工具，并明确暂停、编辑和恢复的预期行为；完成标准应包括达成一致的设计，以及已验证支持在请求发送前对其进行编辑。

由索引模型根据 Issue 内容生成。