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/Credentials/Linux/LinuxInteropTests.cs

@@ -0,0 +1,37 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.ServiceLayer.Credentials;
+using Microsoft.SqlTools.ServiceLayer.Credentials.Linux;
+using Microsoft.SqlTools.ServiceLayer.Test.Utility;
+using Xunit;
+
+namespace Microsoft.SqlTools.ServiceLayer.Test.Credentials
+{
+    public class LinuxInteropTests
+    {
+        [Fact]
+        public void GetEUidReturnsInt()
+        {
+            TestUtils.RunIfLinux(() =>
+            {
+                Assert.NotNull(Interop.Sys.GetEUid());
+            });
+        }
+
+        [Fact]
+        public void GetHomeDirectoryFromPwFindsHomeDir()
+        {
+
+            TestUtils.RunIfLinux(() =>
+            {
+                string userDir = LinuxCredentialStore.GetHomeDirectoryFromPw();
+                Assert.StartsWith("/", userDir);
+            });
+        }
+        
+    }
+}
+