Better description of UDM in Planet conversion / output.

[gadomski]

The new file extension has an example of using a mapping object to describe a cloud cover mask. Seems like it'd be useful to include that information about Planet's cloud mask.

Info at https://developers.planet.com/docs/data/udm-2/ - hrm, it looks like we use one band per class, instead of one band with all classes in it, so perhaps it'd be better to describe it with a 'bands' object, even if there are no common names. Regardless, it'd be good to have more description of the UDM asset in STAC.

Additional notes

Note there have been people who have made single band UDM from Planet's multi-band asset. Could be cool to give an option in planet STAC conversion to reformat into that (arguably) more useful option. Especially if we make a 'band' description for UDM / QA like radiantearth/stac-spec#977

Migration

Migrated from stac-utils/stactools#56.

[cholmes]

Closing this, as the mapping functionality is in flux. stac-extensions/file#6

We probably should upgrade to more of the 'raster' extension stuff, but can make that its own issue.