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/spark/sparkUtils.ts

@@ -0,0 +1,16 @@
+
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+// TODO: The content of this file should be refactored to an extension
+export function getKnoxUrl(host: string, port: string): string {
+	return `https://${host}:${port}/gateway`;
+}
+
+export function getLivyUrl(serverName: string, port: string): string {
+	return getKnoxUrl(serverName, port) + '/default/livy/v1/';
+}