Feature: implement overlay() for .npz source/sink to populate "key" parameter with existing file keys

[ncorgan]

There's some ugliness here regarding when to error out if a given filepath doesn't exist or if a key is invalid, as errors at certain points will prevent PothosFlow from calling the block's overlay() and populating the ComboBox. However, if we solve this, non-PothosFlow callers may not error out at a reasonable point for a good user experience.