determination of border pixels

[johntruckenbrodt]

In the function s1ard.metadata.extract.get_prod_meta a metadata attribute nodata_borderpx is determined by rasterizing the combined footprint of the source scenes, subsetting it to the extent of the ARD product, and counting the number of nodata pixels.
This is inaccurate and more effort than necessary. Instead, the valid data mask created by function s1ard.ancillary.datamask should be used to get the exact valid data footprint of the processed product.

This metadata attribute will be written to the STAC metadata as card4l:border_pixels.