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/Utility/TestUtils.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Microsoft.SqlTools.ServiceLayer.Test.Utility
+{
+    public static class TestUtils
+    {
+
+        public static void RunIfLinux(Action test)
+        {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
+            {
+                test();
+            }
+        }
+        
+        public static void RunIfWindows(Action test)
+        {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            {
+                test();
+            }
+        }
+    }
+}