Add global.json to force Kudu use SDK 2.1.515 (#8)

After .NET Core SDK 3.1.301 was rolled out in Azure App Service, Kudu failed to build the app with the following exception:
System.ComponentModel.Win32Exception (5): Access is denied.

In order to fix the build issue, we add a global.json in the code base. So that Kudu is forced to use the previous SDK version, which can build the app without issue.

See https://stackoverflow.com/questions/62595408/error-access-is-denied-deployment-to-azure-app-services

global.json

@@ -0,0 +1,5 @@
+{
+  "sdk": {	
+	"version": "2.1.515"
+  }
+}