Allow "results" to be uploaded after task run

[brendo]

Our application has a few maintenance tasks which iterate over a collection and then upload a CSV with the results of the task. This is implemented using the after_start/after_complete/after_error callbacks and by flushing to a CSV after each iteration.

What would be neat is if this feature was baked into the gem by default, and the results uploaded was all handled by ActiveStorage and stored on the run model. This would mean that viewing past runs would be able to surface a deep-link to this "results" file.

Is this something that PR's would be accepted for?