Add a file size field to the REST API and use its value when fetching media

This is a follow-up to #6.

The file size is not exposed by default in the WordPress REST API `media` endpoint. This plugin should add a new field to the REST API which exposes it, and then reads that value in `Factory::create()` and sets the file size via `$item->set_file_size()` if the field exists.

This would solve the problem of not having file size information when this plugin is used for sharing media within a Multisite network.

Longer term, we should open a ticket for WordPress core to get the file size added to the REST API too.