Use item assets from the base collection when available

[gjoseph92]

The item assets STAC extension allows collections to describe which assets might be available, and give some metadata common to all instances of those assets—for example, ["eo:bands"]["common_name"]. When this extension is present, if a field is missing on an individual asset, you're supposed to fall back on the field as defined on the item_assets of the parent collection.

This would be particularly handy with #3. Some collections (like Sentinel-2 on AWS) don't provide eo:bands on individual items, just at the collection level, so we're missing some metadata right now.

This is easy to implement; the most annoying part is just handling the "get the parent collection" part across satstac vs pystac vs a plain dict.