Consider supporting an online "upgrade" experience when embedded in an IDE

In this scenario, I opened the Performance page in VS code when no debug sessions were available. This opened up the Performance screen for the "disconnected" state which allows the user to load offline data. I later ran a debug session, and still had the "offline" version of the Performance page.

It would be nice if a small banner popped up at the top of my performance tool that said something like
"A debug session is now available. Would you like to reload this tool and connect to it?". At that point the user could choose whether to ignore the message and continue looking at the offline data or reload the tool with the new app connection.

If the user chose to reload, DevTools could refresh itself so that the embedded IDE view would be reloaded. This would require the IDE to send an event to DevTools debug sessions change, and I think this event already exists because of the Flutter sidebar.

CC @DanTup @helin24

先从 Performance 页面处于断开连接和离线数据状态时的处理开始，然后跟踪 Flutter 侧边栏现有的调试会话变更事件。确定嵌入式 IDE 如何在会话可用时通知 DevTools，以及用户控制的重新加载选择应如何影响当前视图。完成的标准是：处理好这一转换，除非用户选择重新加载，否则不丢弃离线数据。

由索引模型根据 Issue 内容生成。