Method to skip adding core activites automatically.

src/core/Elsa.Core/ElsaOptions.cs

@@ -77,7 +77,14 @@ namespace Elsa
         internal Action<IServiceProvider, JsonSerializer> JsonSerializerConfigurer { get; private set; }
         internal Action AddAutoMapper { get; private set; }
         internal Action<ServiceBusEndpointConfigurationContext> ConfigureServiceBusEndpoint { get; private set; }
-        
+        internal bool WithCoreActivities { get; private set; } = true;
+
+        public ElsaOptions NoCoreActivities()
+        {
+            WithCoreActivities = false;
+            return this;
+        }
+
         public ElsaOptions AddActivity<T>() where T : IActivity => AddActivity(typeof(T));
 
         public ElsaOptions AddActivity(Type activityType)

src/core/Elsa.Core/Extensions/ElsaServiceCollectionExtensions.cs

@@ -191,8 +191,13 @@ namespace Microsoft.Extensions.DependencyInjection
             return options;
         }
 
-        private static ElsaOptions AddCoreActivities(this ElsaOptions services) => services
-            .AddActivitiesFrom<ElsaOptions>()
-            .AddActivitiesFrom<CompositeActivity>();
+        private static ElsaOptions AddCoreActivities(this ElsaOptions services)
+        {
+            if (services.WithCoreActivities)
+                return services
+                    .AddActivitiesFrom<ElsaOptions>()
+                    .AddActivitiesFrom<CompositeActivity>();
+            return services;
+        }
     }
 }