Added small example to prove functionality for receiving masstransit messages

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Controllers/TriggerMessageController.cs

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+using MassTransit;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+using Elsa.Samples.MassTransitRabbitMq.Messages;
+
+namespace Elsa.Samples.MassTransitRabbitMq.Controllers
+{
+    [ApiController]
+    [Route("trigger-message")]
+    public class TriggerMessageController : Controller
+    {
+        private readonly IPublishEndpoint _publishEndpoint;
+
+        public TriggerMessageController(IPublishEndpoint publishEndpoint)
+        {
+            _publishEndpoint = publishEndpoint;
+        }
+
+        [Route("first")]
+        [HttpPost]
+        public async Task<IActionResult> TriggerFirstMessage()
+        {
+            var message = new FirstMessage();
+            await _publishEndpoint.Publish(message);
+            return Ok();
+        }
+
+        [Route("second")]
+        [HttpPost]
+        public async Task<IActionResult> TriggerSecondMessage()
+        {
+            var message = new SecondMessage();
+            await _publishEndpoint.Publish(message);
+            return Ok();
+        }
+    }
+}

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Elsa.Samples.MassTransitRabbitMq.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+    <PropertyGroup>
+        <TargetFramework>net5.0</TargetFramework>
+        <IsPackable>false</IsPackable>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\..\core\Elsa\Elsa.csproj" />
+        <ProjectReference Include="..\..\..\activities\Elsa.Activities.Console\Elsa.Activities.Console.csproj" />
+        <ProjectReference Include="..\..\..\activities\Elsa.Activities.MassTransit\Elsa.Activities.MassTransit.csproj" />
+    </ItemGroup>
+
+</Project>

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Messages/FirstMessage.cs

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+using System;
+
+namespace Elsa.Samples.MassTransitRabbitMq.Messages
+{
+    public class SecondMessage
+    {    
+        public Guid CorrelationId { get; private set; }
+
+        public SecondMessage()
+        {
+            CorrelationId = Guid.Parse("e9ca46dd-36b9-4fc4-b7db-3bb7190e4488");
+        }
+    }
+}

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Messages/SecondMessage.cs

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+using System;
+
+namespace Elsa.Samples.MassTransitRabbitMq.Messages
+{
+    public class FirstMessage
+    {    
+        public Guid CorrelationId { get; private set; }
+
+        public FirstMessage()
+        {
+            CorrelationId = Guid.Parse("e9ca46dd-36b9-4fc4-b7db-3bb7190e4488");
+        }
+    }
+}

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Program.cs

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+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Hosting;
+
+namespace Elsa.Samples.MassTransitRabbitMq
+{
+    public class Program
+    {
+        public static void Main(string[] args) => CreateHostBuilder(args).Build().Run();
+
+        public static IHostBuilder CreateHostBuilder(string[] args) =>
+            Host.CreateDefaultBuilder(args)
+                .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseStartup<Startup>(); });
+    }
+}

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Startup.cs

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+using System;
+using MassTransit;
+using Elsa.Samples.MassTransitRabbitMq.Messages;
+using Elsa.Samples.MassTransitRabbitMq.Workflows;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+using Elsa.Activities.MassTransit.Extensions;
+using Elsa.Activities.MassTransit.Consumers;
+
+namespace Elsa.Samples.MassTransitRabbitMq
+{
+    public class Startup
+    {
+        private Type CreateWorkflowConsumer(Type messageType) => typeof(WorkflowConsumer<>).MakeGenericType(messageType);
+
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddControllers();
+
+            services
+                .AddMassTransit(x =>
+                {
+                    // Add workflow consumer for message
+                    x.AddConsumer(CreateWorkflowConsumer(typeof(FirstMessage)));
+                    x.AddConsumer(CreateWorkflowConsumer(typeof(SecondMessage)));
+
+                    // Configure rabbitmq
+                    x.UsingRabbitMq((ctx, cfg) =>
+                    {
+                        cfg.ConfigureEndpoints(ctx);
+                        cfg.Host("rabbitmq://guest:guest@localhost");
+                    });
+                })
+                .AddMassTransitHostedService();
+
+            services
+                .AddElsa(options => options
+                    .AddConsoleActivities()
+                    .AddMassTransitActivities()
+                    .AddWorkflow<TestWorkflow>()
+                );
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+            app.UseRouting();
+
+            app.UseEndpoints(endpoints =>
+            {
+                endpoints.MapControllers();
+            });
+        }
+    }
+}

src/samples/aspnet/Elsa.Samples.MassTransitRabbitMq/Workflows/TestWorkflow.cs

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+using Elsa.Activities.Console;
+using Elsa.Activities.MassTransit;
+using Elsa.Activities.Primitives;
+using Elsa.Builders;
+using Elsa.Samples.MassTransitRabbitMq.Messages;
+using Elsa.Activities.ControlFlow;
+using System;
+
+namespace Elsa.Samples.MassTransitRabbitMq.Workflows
+{
+    public class TestWorkflow : IWorkflow
+    {
+        public void Build(IWorkflowBuilder builder)
+        {
+            builder
+                .ReceiveMassTransitMessage(
+                    activity => activity.Set(x => x.MessageType, x => typeof(FirstMessage))
+                )
+                .Correlate(activity => activity.Set(x => x.Value, context => context.GetInput<FirstMessage>().CorrelationId.ToString()))
+                .WriteLine(context => $"Received first message")
+
+                // Wait until second message received with the same correlation id
+                .ReceiveMassTransitMessage(
+                    activity => activity.Set(x => x.MessageType, x => typeof(SecondMessage))
+                )
+                .WriteLine(context => $"Received second message");
+        }
+    }
+}