Convenience method for downloading assets via stac-asset

[jsignell]

stac-asset implements patterns like:

import pystac
import stac_asset

href = "https://raw.githubusercontent.com/radiantearth/stac-spec/master/examples/simple-item.json"
item = pystac.read_file(href)
await stac_asset.download_item(item, ".")

Seems like there would be some ergonomics and discoverability wins in adding a download method to pystac.Item itself so you could do:

import pystac

href = "https://raw.githubusercontent.com/radiantearth/stac-spec/master/examples/simple-item.json"
item = pystac.read_file(href)
await item.download(".")