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/contracts.ts

@@ -0,0 +1,47 @@
+
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+export type CellType = 'code' | 'markdown' | 'raw';
+
+export class CellTypes {
+	public static readonly Code = 'code';
+	public static readonly Markdown = 'markdown';
+	public static readonly Raw = 'raw';
+}
+
+// to do: add all mime types
+export type MimeType = 'text/plain' | 'text/html';
+
+// to do: add all mime types
+export class MimeTypes {
+	public static readonly PlainText = 'text/plain';
+	public static readonly HTML = 'text/html';
+}
+
+export type OutputType =
+	| 'execute_result'
+	| 'display_data'
+	| 'stream'
+	| 'error'
+	| 'update_display_data';
+
+export class OutputTypes {
+	public static readonly ExecuteResult = 'execute_result';
+	public static readonly DisplayData = 'display_data';
+	public static readonly Stream = 'stream';
+	public static readonly Error = 'error';
+	public static readonly UpdateDisplayData = 'update_display_data';
+}
+
+export enum NotebookChangeType {
+	CellsAdded,
+	CellDeleted,
+	CellSourceUpdated,
+	CellOutputUpdated,
+	DirtyStateChanged
+}