Enable hostcontext to track auth migration. (#3776)

src/Runner.Common/AuthMigration.cs

@@ -0,0 +1,13 @@
+using System;
+
+namespace GitHub.Runner.Common
+{
+    public class AuthMigrationEventArgs : EventArgs
+    {
+        public AuthMigrationEventArgs(string trace)
+        {
+            Trace = trace;
+        }
+        public string Trace { get; private set; }
+    }
+}