Port most notebook model code over to be behind a service (#3068)

- Defines a new NotebookService in Azure Data Studio which will be used to interact with notebooks. Since notebooks can require per-file instantiation the provider is just used to create & track managers for a given URI.
- Inject this into notebook.component.ts and pass required parameters that'll be used to properly initialize a manger into the method. Actual initialization not done yet.
- Port over & recompile notebook model code
- Define most required APIs in sqlops.proposed.d.ts. In the future, these will be used by extensions to contribute their own providers.

src/sql/parts/notebook/models/modelFactory.ts

@@ -0,0 +1,23 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import { nb } from 'sqlops';
+
+import { CellModel } from './cell';
+import { IClientSession, IClientSessionOptions, ICellModelOptions, ICellModel, IModelFactory } from './modelInterfaces';
+import { ClientSession } from './clientSession';
+
+export class ModelFactory implements IModelFactory {
+
+	public createCell(cell: nb.ICell, options: ICellModelOptions): ICellModel {
+		return new CellModel(this, cell, options);
+	}
+
+	public createClientSession(options: IClientSessionOptions): IClientSession {
+		return new ClientSession(options);
+	}
+}