Add "rooted" file name strategy

[gadomski]

In this strategy, a common root path would be found for all assets, and that common root would be removed from all local paths. E.g., if the asset hrefs are:

• http://stac-asset.test/assets/a/sub/directory/file.tif
• http://stac-asset.test/assets/a/nother/sub/directory/file.tif

The local paths, after download, would be:

• sub/directory/file.tif
• nother/sub/directory/file.tif

This strategy is useful if multiple assets have the same file name and you don't want to use the key strategy, or if the assets are expected to have some relative file path structure (e.g. for sentinel granules). This strategy is BAD if assets are in different locations (e.g. http://stac-asset.test/file.tif and http://stac-asset-2.test/file.tif) and we should probably error in this case.