[ENH] About the Elsa.Workflows.Pipelines.ActivityExecution

Suggest aligning the `ActiveExecution` programming paradigm with the `WorkflowExecution`, for example:

- `ActiveExecutionPipeBuilder` suggests renaming it to `ActiveExecutionPipeBuilder`
- Add the ActiveExecutionMiddleware class with the following code:

```csharp
namespace Elsa.Workflows.Pipelines.ActivityExecution;

public abstract class ActivityExecutionMiddleware : IActivityExecutionMiddleware
{
protected ActivityExecutionMiddleware(ActivityMiddlewareDelegate next) => Next = next;
protected ActivityMiddlewareDelegate Next { get; }
public abstract ValueTask InvokeAsync(ActivityExecutionContext context);
}
```