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

test/Microsoft.SqlTools.ServiceLayer.Test/QueryExecution/DisposeTests.cs

@@ -7,6 +7,7 @@ using System;
 using System.Threading.Tasks;
 using Microsoft.SqlTools.ServiceLayer.Hosting.Protocol;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
+using Microsoft.SqlTools.ServiceLayer.Test.Utility;
 using Moq;
 using Xunit;
 
@@ -21,7 +22,7 @@ namespace Microsoft.SqlTools.ServiceLayer.Test.QueryExecution
             // ... I request a query (doesn't matter what kind)
             var queryService = Common.GetPrimedExecutionService(Common.CreateMockFactory(null, false), true);
             var executeParams = new QueryExecuteParams {QueryText = "Doesn'tMatter", OwnerUri = Common.OwnerUri};
-            var executeRequest = Common.GetQueryExecuteResultContextMock(null, null, null);
+            var executeRequest = RequestContextMocks.SetupRequestContextMock<QueryExecuteResult, QueryExecuteCompleteParams>(null, QueryExecuteCompleteEvent.Type, null, null);
             queryService.HandleExecuteRequest(executeParams, executeRequest.Object).Wait();
 
             // ... And then I dispose of the query