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/Win32/CredentialResources.cs

@@ -0,0 +1,16 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+namespace Microsoft.SqlTools.ServiceLayer.Credentials.Win32
+{
+    // TODO Replace this strings class with a resx file
+    internal class CredentialResources
+    {
+        public const string PasswordLengthExceeded = "The password has exceeded 512 bytes.";
+        public const string TargetRequiredForDelete = "Target must be specified to delete a credential.";
+        public const string TargetRequiredForLookup = "Target must be specified to check existance of a credential.";
+        public const string CredentialDisposed = "Win32Credential object is already disposed.";
+    }
+}