Add back Azure Resource Explorer extension with updated build script (#2805)

* Revert "Revert "Port the Azure Resource Explorer extension to core." (#2770)"

This reverts commit 210447cd37.

* WIP1

* Add custom build task for azurecore extension

* Fix azurecore output path for macOS build

* Fix linux gulp task name

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();
+}