SQL Agent configuration for Operators, Alerts and Proxies (WIP) (#621)

* Initial non-refactored SQL Agent alert classes (WIP)

* Move agent classes into subdirectories

* Refactor the agent config code a bit more

* Add more implementation for handlers

* Add more code to the create alert handler

* Clean up agent alert class

* Clean up alert methods a bit

* Initial Operator contracts

* Additonal SQL Agent config changes

* More Proxy config cleanup

* Cleanup AgentProxy class

* Additional cleanups

* Run SRGen

* Add security service to create credential objects

src/Microsoft.SqlTools.ServiceLayer/Security/Contracts/CredentialInfo.cs

@@ -0,0 +1,23 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Collections.Generic;
+using Microsoft.SqlTools.ServiceLayer.Agent;
+
+namespace Microsoft.SqlTools.ServiceLayer.Security.Contracts
+{
+    /// <summary>
+    /// a class for storing various credential properties
+    /// </summary>
+    public class CredentialInfo
+    {
+        public string Identity { get; set; }
+        public int Id { get; }
+        public DateTime DateLastModified { get; }
+        public DateTime CreateDate { get; }
+       public string ProviderName { get; set; }       
+    }
+}