[Identity] Fix sample app (#6805)

Updates the sample app to account for changes in how MVC uses JSON.NET

src/Identity/ApiAuthorization.IdentityServer/samples/ApiAuthSample/ApiAuthSample.csproj

@@ -16,6 +16,7 @@
     <Reference Include="Microsoft.AspNetCore.Identity.EntityFrameworkCore" />
     <Reference Include="Microsoft.AspNetCore.Identity.UI" />
     <Reference Include="Microsoft.AspNetCore.Mvc" />
+    <Reference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" />
     <Reference Include="Microsoft.AspNetCore.Rewrite" />
     <Reference Include="Microsoft.AspNetCore.Server.IISIntegration" />
     <Reference Include="Microsoft.AspNetCore.Server.Kestrel" />

src/Identity/ApiAuthorization.IdentityServer/samples/ApiAuthSample/Startup.cs

@@ -1,4 +1,4 @@
-using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Mvc;
@@ -34,7 +34,8 @@ namespace ApiAuthSample
             services.AddAuthentication()
                 .AddIdentityServerJwt();
 
-            services.AddMvc();
+            services.AddMvc()
+                .AddNewtonsoftJson();
         }
 
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.