Add solar_illumination to Band object

The [Band Object](https://github.com/stac-extensions/eo#band-object) in the EO spec includes a `solar_illumination` field, but we don't currently provide this in the [`Band` class](https://github.com/stac-utils/pystac/blob/main/pystac/extensions/eo.py#L29).