Create Firewall Rule support with a simple Resource Provider implementation

Implementation of the resource provider APIs in order to support Create Firewall Rule. Provides definition for a ResourceProvider and Authentication service. The ResourceProvider supports firewall rules for now, and since authentication is routed through that method it will call into the auth service to set up the current account to be used.

Additional notes:
- Fixed deserialization by adding an Accept header. This shouldn't be necessary, but for some reason the firewall rule defaults to XML without this
- Use generic server list and parse the ID to get the resource group, avoiding a large number of extra calls for each RG
- Errors now include error message from the API

test/Microsoft.SqlTools.ServiceLayer.UnitTests/ResourceProvider/FirewallErrorParserTest.cs

@@ -3,7 +3,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System;
-using Microsoft.SqlTools.ResourceProvider.Core.FirewallRule;
+using Microsoft.SqlTools.ResourceProvider.Core.Firewall;
 using Xunit;
 
 namespace Microsoft.SqlTools.ServiceLayer.UnitTests.ResourceProvider