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/Credentials/Linux/Interop.Sys.cs

@@ -0,0 +1,42 @@
+//
+// 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.Runtime.InteropServices;
+
+namespace Microsoft.SqlTools.ServiceLayer.Credentials
+{
+    internal static partial class Interop
+    {
+        internal static partial class Sys
+        {
+            [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_ChMod", SetLastError = true)]
+            internal static extern int ChMod(string path, int mode);
+            
+            internal struct Passwd
+            {
+                internal IntPtr Name;           // char*
+                internal IntPtr Password;       // char*
+                internal uint  UserId;
+                internal uint  GroupId;
+                internal IntPtr UserInfo;       // char*
+                internal IntPtr HomeDirectory;  // char*
+                internal IntPtr Shell;          // char*
+            };
+
+            [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_GetPwUidR", SetLastError = false)]
+            internal static extern int GetPwUidR(uint uid, out Passwd pwd, IntPtr buf, int bufLen);
+
+            [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_GetEUid")]
+            internal static extern uint GetEUid();
+
+            private static partial class Libraries
+            {
+                internal const string SystemNative = "System.Native";
+            }
+        }
+        
+    }
+}