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/azureResource/servicePool.ts

@@ -0,0 +1,35 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 {
+	IAzureResourceAccountService,
+	IAzureResourceCredentialService,
+	IAzureResourceSubscriptionService,
+	IAzureResourceSubscriptionFilterService,
+	IAzureResourceDatabaseService,
+	IAzureResourceDatabaseServerService,
+	IAzureResourceCacheService,
+	IAzureResourceContextService } from './interfaces';
+
+export class AzureResourceServicePool {
+	private constructor() { }
+
+	public static getInstance(): AzureResourceServicePool {
+		return AzureResourceServicePool._instance;
+	}
+
+	public contextService: IAzureResourceContextService;
+	public cacheService: IAzureResourceCacheService;
+	public accountService: IAzureResourceAccountService;
+	public credentialService: IAzureResourceCredentialService;
+	public subscriptionService: IAzureResourceSubscriptionService;
+	public subscriptionFilterService: IAzureResourceSubscriptionFilterService;
+	public databaseService: IAzureResourceDatabaseService;
+	public databaseServerService: IAzureResourceDatabaseServerService;
+
+	private static readonly _instance = new AzureResourceServicePool();
+}