refactor: routing convention implements controller mapping interface

Author: maurei

src/JsonApiDotNetCore/Builders/JsonApiApplicationBuilder.cs

@@ -73,7 +73,7 @@ public void ConfigureMvc()
 
             var routingConvention = intermediateProvider.GetRequiredService<IJsonApiRoutingConvention>();
             _mvcBuilder.AddMvcOptions(opt => opt.Conventions.Insert(0, routingConvention));
-            _services.AddSingleton((IControllerResourceMapping)routingConvention); 
+            _services.AddSingleton<IControllerResourceMapping>(routingConvention); 
         }
 
         /// <summary>

src/JsonApiDotNetCore/Internal/IJsonApiRoutingConvention.cs

@@ -6,5 +6,5 @@ namespace JsonApiDotNetCore.Internal
     /// Service for specifying which routing convention to use. This can be overriden to customize
     /// the relation between controllers and mapped routes.
     /// </summary>
-    public interface IJsonApiRoutingConvention : IApplicationModelConvention { }
+    public interface IJsonApiRoutingConvention : IApplicationModelConvention, IControllerResourceMapping { }
 }