Restart kernel initial implementation (#18835)

* Restart kernel initial implementation

* Update notebook extension TestKernel

* PR comments

src/sql/azdata.proposed.d.ts

@@ -128,6 +128,24 @@ declare module 'azdata' {
 		 * An event that is emitted when a [notebook document](#NotebookDocument) is closed.
 		 */
 		export const onDidCloseNotebookDocument: vscode.Event<NotebookDocument>;
+
+		export interface IKernel {
+
+			/**
+			 * Restart a kernel.
+			 *
+			 * #### Notes
+			 * Uses the [Jupyter Notebook API](http://petstore.swagger.io/?url=https://raw.githubusercontent.com/jupyter/notebook/4.x/notebook/services/api/api.yaml#!/kernels).
+			 *
+			 * The promise is fulfilled on a valid response and rejected otherwise.
+			 *
+			 * It is assumed that the API call does not mutate the kernel id or name.
+			 *
+			 * The promise will be rejected if the kernel status is `Dead` or if the
+			 * request fails or the response is invalid.
+			 */
+			restart(): Thenable<void>;
+		}
 	}
 
 	/**