Extend observer with onFileStage

[rcannood]

Closes #5905

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[bentsherman]

Before we go down this road, we should see if we can simply preserve the remote path here:

nextflow/modules/nextflow/src/main/groovy/nextflow/processor/TaskProcessor.groovy

Line 1943 in 36153c6

def holder = new FileHolder(target)

Then you should be able to use the onProcessPending event in the trace observer to get the original path via handler.task.inputs. That would be preferable to adding a new trace event

[rcannood]

Before we go down this road, we should see if we can simply preserve the remote path here:

Added to #5911 -- not sure whether this is what you had in mind.

Having said that, I wonder whether adding this function to the observer isn't also just a good addition, because it would make it super clear from the documentation how to get observe staging events.