Create a way for users to try beta or master DevTools without having to switch their flutter version

Since we ship DevTools with the flutter SDK, we often run into a latency issue where we have shipped a DevTools version on Flutter master that has bug fixes users are interested in (e.g. the network page fixes of recent), but users have no way to try this DevTools version out unless they switch to the flutter master channel. This can be problematic for users, as they may not be able to switch to the master channel without a big lift.

We should design a way for users to try DevTools from Flutter Beta or Flutter master at the click of a button in DevTools. We will not be able to guarantee that this DevTools version will be compatible with the user's Flutter version, but we can warn users about this.