[branch-connections]: auto-start LiveShare session when switching to a connected feature branch

[legomushroom]

When we connect a branch and, at some point, user switches to that branch, we show "do you want to start the session" notification:

There should be a setting instead that allows controlling the behavior:

• Notification(current and default): shows a notification with Settings and Start session buttons.
• Auto-start: starts the session as soon as the user switches to the branch, no notification is shown.
• Silent: no notification is shown, the sessions do not start automatically.