Added Unified connection support (#3785)

* Added Unified connection support

* Use generic way to do expandNode.
Cleanup the ported code and removed unreference code. Added as needed later.
Resolved PR comments.

* Minor fixes and removed timer for all expanders for now. If any providers can't response, the tree node will spin and wait. We may improve later.

* Change handSessionClose to not thenable.
Added a node to OE to show error message instead of reject. So we could show partial expanded result if get any.
Resolve PR comments

* Minor fixes of PR comments

src/sqltest/stubs/connectionManagementService.test.ts

@@ -254,6 +254,10 @@ export class TestConnectionManagementService implements IConnectionManagementSer
 		return undefined;
 	}
 
+	getServerInfo(profileId: string): sqlops.ServerInfo {
+		return undefined;
+	}
+
 	getConnectionString(connectionId: string): Thenable<string> {
 		return undefined;
 	}