DLL discovery feature

I reference all elsa features as NuGet packages:

And then I need to define them in my startup like this:
```csharp
var startups = new[]
{
typeof(Elsa.Activities.Console.Startup),
...
typeof(Webhooks.Persistence.YesSql.MySqlStartup),
typeof(Webhooks.Persistence.YesSql.PostgreSqlStartup),
};

services
.AddElsa(options => options
.AddActivitiesFrom()
.AddWorkflowsFrom()
.AddFeatures(new[] { typeof(Startup) }, Configuration)
// this line should be obsolete!
.AddFeatures(startups, Configuration)
```

Can we get rid of `.AddFeatures(startups, Configuration)` and instead search for all the libraries automatically?

I did something like this before in another project:
```csharp
ServiceAssemblies = Directory.EnumerateFiles(serviceDir, "*.dll", SearchOption.TopDirectoryOnly)
.Select(Assembly.LoadFrom)
.Where(_ => !_.ManifestModule.Name.StartsWith("ServiceDefinitions.dll"))
.ToArray();

builder.RegisterAssemblyTypes(ServiceAssemblies)
.Where(t => t.IsAssignableTo())
.Where(t => !t.IsAbstract)
.As()
.AsSelf()
.InstancePerDependency();
```

Start by tracing the existing AddFeatures(startups, Configuration) registration path alongside AddActivitiesFrom() and AddWorkflowsFrom(). Review the referenced assembly-loading approach using Directory.EnumerateFiles and Assembly.LoadFrom, then determine the supported discovery scope and configuration behavior. Done means feature assemblies are discovered without manually listing startup types and existing registration still works.

Written by the indexing model from the issue text.