Add option to write out residual shower model from straylight step

[stscijgbot-jp]

Issue JP-3905 was created on JIRA by David Law:

Recent work on https://jira.stsci.edu/browse/JP-3830 added the ability for the straylight step to create and subtract a model of residual cosmic ray showers from MIRI MRS data using the --clean_showers=True argument.  Feedback from users since this was integrated into the pipeline has indicated that it would be useful to be able to pass an option to write out the shower model that is being subtracted from the data in order to inspect the performance of the routine.

This would be an off-by-default option, akin to the ability of the clean_flicker_noise step to write out a variety of optional outputs.