Port the Azure Resource Explorer extension to core. (#2701)

* Port the Azure Resource Explorer extension to core.

This will enable Azure viewlet by default in the next release.

- Moving this code from the SQL Server 2019 extension to Azure Data Studio core
- Ported tests and verified they work in the integration tests.sh file
- Fixed an issue that caused integration tests to fail if you have a SQL Server 2019 big data cluster endpoint listed, but the extension isn't installed.

extensions/azurecore/src/controllers/controllerBase.ts

@@ -0,0 +1,34 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 * as vscode from 'vscode';
+
+import { AppContext } from '../appContext';
+import { ApiWrapper } from '../apiWrapper';
+
+export default abstract class ControllerBase implements vscode.Disposable {
+
+	public constructor(protected appContext: AppContext) {
+	}
+
+	protected get apiWrapper(): ApiWrapper {
+		return this.appContext.apiWrapper;
+	}
+
+	public get extensionContext(): vscode.ExtensionContext {
+		return this.appContext && this.appContext.extensionContext;
+	}
+
+	abstract activate(): Promise<boolean>;
+
+	abstract deactivate(): void;
+
+	public dispose(): void {
+		this.deactivate();
+	}
+}
+