Credentials store API (#38)

* CredentialService initial impl with Win32 support

- Basic CredentialService APIs for Save, Read, Delete
- E2E unit tests for Credential Service
- Win32 implementation with unit tests

* Save Password support on Mac v1

- Basic keychain support on Mac using Interop with the KeyChain APIs
- All but 1 unit test passing. This will pass once API is changed, but checking this in with the existing API so that if we decide to alter behavior, we have a reference point.

* Remove Username from Credentials API

- Removed Username option from Credentials as this caused conflicting behavior on Mac vs Windows

* Cleanup Using Statements and add Copyright

* Linux CredentialStore Prototype

* Linux credential store support

- Full support for Linux credential store with tests

* Plumbed CredentialService into Program init

* Addressing Pull Request comments

src/Microsoft.SqlTools.ServiceLayer/Program.cs

@@ -2,19 +2,17 @@
 // 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.Threading.Tasks;
 using Microsoft.SqlTools.EditorServices.Utility;
 using Microsoft.SqlTools.ServiceLayer.Hosting;
 using Microsoft.SqlTools.ServiceLayer.SqlContext;
 using Microsoft.SqlTools.ServiceLayer.Workspace;
 using Microsoft.SqlTools.ServiceLayer.LanguageServices;
 using Microsoft.SqlTools.ServiceLayer.Connection;
-using Microsoft.SqlTools.ServiceLayer.Connection.Contracts;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution;
-using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
+using Microsoft.SqlTools.ServiceLayer.Credentials;
 
 namespace Microsoft.SqlTools.ServiceLayer
-{     
+{
     /// <summary>
     /// Main application class for SQL Tools API Service Host executable
     /// </summary>
@@ -50,6 +48,7 @@ namespace Microsoft.SqlTools.ServiceLayer
             AutoCompleteService.Instance.InitializeService(serviceHost);
             LanguageService.Instance.InitializeService(serviceHost, sqlToolsContext);
             ConnectionService.Instance.InitializeService(serviceHost);
+            CredentialService.Instance.InitializeService(serviceHost);
             QueryExecutionService.Instance.InitializeService(serviceHost);
 
             serviceHost.Initialize();