User data sync: Implement starting flow

- user identity
- user login provider
- user data provider
- activation event
- status bar UI

src/vs/workbench/api/browser/extensionHost.contribution.ts

@@ -18,6 +18,7 @@ import './mainThreadCodeInsets';
 import './mainThreadClipboard';
 import './mainThreadCommands';
 import './mainThreadConfiguration';
+import './mainThreadUserData';
 import './mainThreadConsole';
 import './mainThreadDebugService';
 import './mainThreadDecorations';