how can I get selected indices from nonMaximumSuppression feature engine?

## ❓Question

I am trying to add NMS in a pipeline, however, I only see `confidenceOutputFeatureName` and `coordinatesOutputFeatureName`, I cannot find any option to output the selected indices.

Start by reviewing the nonMaximumSuppression feature engine options and the existing confidenceOutputFeatureName and coordinatesOutputFeatureName behavior. Determine whether selected indices are supported as an output and what configuration or API change would be required; done means the capability or its limitation is clearly established.

Written by the indexing model from the issue text.