Support relative asset links

[gjoseph92]

I'm sure we don't handle it correctly right now. This will be necessary to work with any self-contained catalog or relative published catalog, which could be particularly useful if working with local data.

There's perhaps a larger question here about whether we want to use PySTAC internally instead of raw JSON, since it would handle this (and other things, like item assets #4) for us.

[maawoo]

For anyone who wants to bypass this issue without having to adjust the STAC Catalog on disk:

import pystac
import stackstac

stac_catalog = pystac.Catalog.from_file('path/to/my/catalog.json')
stac_catalog.make_all_asset_hrefs_absolute() # only in-memory
ds = stackstac.stack(items=stac_catalog)