Begin defining Extension-based Notebook Provider (#3172)

Implements provider contribution in the MainThreadNotebook, with matching function calls in the ExtHostNotebook class. This will allow us to proxy through notebook providers (specifically, creation of a notebook manager with required content, server managers) from an extension up through to the main process.

Implemented in this PR:
- Callthroughs for content and server manager APIs
- Very basic unit tests covering provider & manager registration

Not implemented:
- Fuller unit tests on the specific callthrough methods for content & server manager.
- Contribution point needed to test this (so we can actually pass through the extension's existing Notebook implementation)

src/sql/sqlops.proposed.d.ts

@@ -1430,19 +1430,19 @@ declare module 'sqlops' {
 			/* Reads contents from a Uri representing a local or remote notebook and returns a
 			 * JSON object containing the cells and metadata about the notebook
 			 */
-			getNotebookContents(path: string): Thenable<INotebook>;
+			getNotebookContents(notebookUri: vscode.Uri): Thenable<INotebook>;
 
 			/**
 			 * Save a file.
 			 *
-			 * @param path - The desired file path.
+			 * @param notebookUri - The desired file path.
 			 *
 			 * @param notebook - notebook to be saved.
 			 *
 			 * @returns A thenable which resolves with the file content model when the
 			 *   file is saved.
 			 */
-			save(path: string, notebook: INotebook): Thenable<INotebook>;
+			save(notebookUri: vscode.Uri, notebook: INotebook): Thenable<INotebook>;
 		}
 
 		export interface INotebook {