Fix backup service to have multiple backup instances (#396)

* Allow multiple backups per backup service instance

* Add test to run multiple backups

* Update backup cancelTask method signature

* Fix to have multiple backup instances and add more tests

* Address PR comments

* Remove double new lines

* Add Azure check for backup operations

src/Microsoft.SqlTools.ServiceLayer/DisasterRecovery/IBackupOperation.cs

@@ -0,0 +1,47 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+using Microsoft.SqlServer.Management.Smo;
+using Microsoft.SqlTools.ServiceLayer.Admin;
+using Microsoft.SqlTools.ServiceLayer.DisasterRecovery.Contracts;
+using System.Data.SqlClient;
+
+namespace Microsoft.SqlTools.ServiceLayer.DisasterRecovery
+{
+    /// <summary>
+    /// Interface for backup operations
+    /// </summary>
+    public interface IBackupOperation
+    {
+        /// <summary>
+        /// Initialize 
+        /// </summary>
+        /// <param name="dataContainer"></param>
+        /// <param name="sqlConnection"></param>
+        void Initialize(CDataContainer dataContainer, SqlConnection sqlConnection);
+
+        /// <summary>
+        /// Return database metadata for backup
+        /// </summary>
+        /// <param name="databaseName"></param>
+        /// <returns></returns>
+        BackupConfigInfo GetBackupConfigInfo(string databaseName);
+
+        /// <summary>
+        /// Set backup input properties
+        /// </summary>
+        /// <param name="input"></param>
+        void SetBackupInput(BackupInfo input);
+        
+        /// <summary>
+        /// Execute backup
+        /// </summary>
+        void PerformBackup();
+        
+        /// <summary>
+        /// Cancel backup
+        /// </summary>
+        void CancelBackup();
+    }
+}