Make it possible to publish `shorebird` changes to specific devs but not all (e.g. channels)

[eseidel]

I suspect the "easy" solution here is to just match what Flutter does and use git branches as "channels". This would mean adding a shorebird channel command like flutter channel and making that call git checkout $branch as well as teaching the shorebird --version command to print the current "channel".

flutter_tools has a bunch of code for this I suspect we could take inspiration from.

[eseidel]

https://github.com/shorebirdtech/shorebird/issues/181 is also related.

[eseidel]

Dupe of https://github.com/shorebirdtech/shorebird/issues/110.

[eseidel]

Actually this isn't a dupe of #110. #110 is about staged rollouts of changes to your flutter app. This is about staged rollout of shorebird itself.

[eseidel]

"What command do I run at the command line to know that I'm on "shorebird main", and if so how do I switch back to stable?"

git -C $HOME/.shorebird branch
git -C $HOME/.shorebird checkout stable

Is the current method. :)

[eseidel]

This hasn't turned out to be needed.