We need a way to start the setup flow but using GHE.com instead of GitHub.com

[TylerLeonhardt]

Continuing on from https://github.com/microsoft/vscode/pull/237993

We need the new Core contributed Sign up view to allow a GHE.com user to sign in. Right now, there is only one button and it only signs in with GitHub.com.

Spitballing ideas... maybe we have the drop down (showing GHE.com login) just as we do for when the user has Copilot installed but isn't logged in... but for the GHE flow, we could install the extension first then run the GHE specific auth command contributed by Copilot Chat.