[Feature]: Add regional subsetting capabilities to precip variability metrics

[acordonez]

Is your feature request related to a problem?

Currently the precipitation variability metrics compute metrics globally and for a predefined set of regions. However, it would be useful to have flexibility in defining regions. Also, for high resolution data, the driver can run into memory issues when running globally.

Describe the solution you'd like

Add options to select regions by shapefile and by lat/lon block.

Describe alternatives you've considered

No response

Additional context

This capability will support running regionally downscaled data in the PMP