Allow creation of variables dynamically via setVariable (#4800)

* Disable MassTransitRabbitMq in Elsa.Server.Web.

* Create variable if not exists in Workflow.Core

The update introduced checks if a variable exists before attempting to set its value in the ExpressionExecutionContextExtensions. If the variable doesn't exist, it now creates one automatically.

Fixes #4789

* Remove unnecessary comment

The comment indicating to create a variable if it doesn't exist was deemed superfluous and removed from the ExpressionExecutionContextExtensions.cs file. This improves code cleanliness as the functionality is self-explanatory through the existing code structure.

src/bundles/Elsa.Server.Web/Program.cs

@@ -28,7 +28,7 @@
 const bool useQuartz = true;
 const bool useMassTransit = true;
 const bool useMassTransitAzureServiceBus = false;
-const bool useMassTransitRabbitMq = true;
+const bool useMassTransitRabbitMq = false;
 
 var builder = WebApplication.CreateBuilder(args);
 var services = builder.Services;

src/modules/Elsa.Workflows.Core/Extensions/ExpressionExecutionContextExtensions.cs

@@ -292,7 +292,12 @@ where v.TryGet(context, out _)
             select v;
 
         var variable = q.FirstOrDefault();
-        variable?.Set(context, value);
+
+        if(variable != null)
+            variable.Set(context, value);
+
+        if (variable == null)
+            CreateVariable(context, variableName, value);
     }
 
     /// <summary>