V2: Reading io.read_file, custom reader is not set on object

Looks like when using the top level `read_file` with a custom reader, the custom reader is not propagated down to the object. This means that then calling `get_items()`, for example, the target will be read with the standard default reader. It looks like STACObject has a [from_file](https://github.com/stac-utils/pystac/blob/1a37397982f346277373b565db607cc229fb57cd/src/pystac/stac_object.py#L92) method that handles setting a reader on the object that could be used