Support file info extension on import / item post

### Improvement

I think we have enough information in import (catalog and item) and item post to get file extension info. In import, since it's costly, we could let users opt in, and in item post we could handle it in the background.

### Notes + Context

See https://github.com/radiantearth/stac-spec/pull/934

Start by locating the import paths for catalogs and items and the item POST handling, then read the linked STAC specification pull request for the intended file-extension information. Done should include opt-in support for the costly import behavior and background handling for item POSTs, with the resulting file information exposed as described by the extension.

Written by the indexing model from the issue text.