Support modeling status on pipeline steps

Pipeline steps don't currently support modeling their status. We should add this so we can track if steps are waiting, running, or completed. The status should also capture completion state.

- Introduce a new PipelineStepStatus enum.
```
enum PipelineStepStatus {
Pending,
Running,
Succeeded,
Failed,
Canceled
}
```
- Add a `Status` property to a PipelineStep
- Update the DistributedApplicationPipeline to set the status on each of those steps