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/notebook.component.ts

@@ -129,7 +129,7 @@ export class NotebookComponent extends AngularDisposable implements OnInit {
 		this.notebookManager = await this.notebookService.getOrCreateNotebookManager(this.notebookParams.providerId, this.notebookParams.notebookUri);
 		let model = new NotebookModel({
 			factory: this.modelFactory,
-			path: this.notebookParams.notebookUri.fsPath,
+			notebookUri: this.notebookParams.notebookUri,
 			connectionService: this.connectionManagementService,
 			notificationService: this.notificationService,
 			notebookManager: this.notebookManager