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/parts/notebook/models/modelInterfaces.ts

@@ -21,7 +21,7 @@ import { NotebookConnection } from 'sql/parts/notebook/models/notebookConnection
 import { IConnectionManagementService } from 'sql/parts/connection/common/connectionManagement';
 
 export interface IClientSessionOptions {
-	path: string;
+	notebookUri: URI;
 	notebookManager: INotebookManager;
 	notificationService: INotificationService;
 }
@@ -73,7 +73,7 @@ export interface IClientSession extends IDisposable {
 	/**
 	 * The current path associated with the client session.
 	 */
-	readonly path: string;
+	readonly notebookUri: URI;
 
 	/**
 	 * The current name associated with the client session.
@@ -354,7 +354,7 @@ export interface INotebookModelOptions {
 	/**
 	 * Path to the local or remote notebook
 	 */
-	path: string;
+	notebookUri: URI;
 
 	/**
 	 * Factory for creating cells and client sessions