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/Hosting/Protocol/IProtocolEndpoint.cs

@@ -1,6 +1,9 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
+//
+// 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.ServiceLayer.Hosting.Protocol.Contracts;
 
@@ -13,17 +16,17 @@ namespace Microsoft.SqlTools.ServiceLayer.Hosting.Protocol
     /// </summary>
     public interface IProtocolEndpoint : IMessageSender
     {
-        void SetRequestHandler<TParams, TResult>(
-            RequestType<TParams, TResult> requestType,
+        void SetRequestHandler<TParams, TResult>(
+            RequestType<TParams, TResult> requestType,
             Func<TParams, RequestContext<TResult>, Task> requestHandler);
 
-        void SetEventHandler<TParams>(
-            EventType<TParams> eventType,
+        void SetEventHandler<TParams>(
+            EventType<TParams> eventType,
             Func<TParams, EventContext, Task> eventHandler);
 
-        void SetEventHandler<TParams>(
-            EventType<TParams> eventType,
-            Func<TParams, EventContext, Task> eventHandler,
+        void SetEventHandler<TParams>(
+            EventType<TParams> eventType,
+            Func<TParams, EventContext, Task> eventHandler,
             bool overrideExisting);
     }
 }

src/Microsoft.SqlTools.ServiceLayer/Hosting/Protocol/MessageReader.cs

@@ -11,7 +11,6 @@ using System.Threading.Tasks;
 using Microsoft.SqlTools.EditorServices.Utility;
 using Microsoft.SqlTools.ServiceLayer.Hosting.Protocol.Contracts;
 using Microsoft.SqlTools.ServiceLayer.Hosting.Protocol.Serializers;
-using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 
 namespace Microsoft.SqlTools.ServiceLayer.Hosting.Protocol