Support ParallelForEach api for scheduling workflow items

Examples:
```cs
public class TranscodeWorkflow : Workflow
{
public TranscodeWorkflow()
{
ScheduleActivity();
ParallelForEach(_=>new {"MPEG", "WAV"}).ScheduleActivity()
.WithInput(i=>new {Path =p.ParentResult().Path, Format= i.Key})
.AfterActivity()
}
}
```

```cs
public class ProcessImagesWorkflow : Workflow
{
public TranscodeWorkflow()
{
ScheduleActivity();
ParallelForEach(a=>a.ParentResult().Images, limit=10).ScheduleActivity()
.WithInput(i=>new {Path =i.Key})
.AfterActivity()
}
}
```

Start by reviewing the two ParallelForEach examples in the issue alongside the existing workflow scheduling API. Define how collection results, item inputs, and the optional concurrency limit should behave; done means both shown workflow patterns are supported and can schedule their activities as described.

Written by the indexing model from the issue text.