Add filters in RSS/Atom feeds

Currently, it is possible to have a feed with either:
- All new packages
- All new versions
- All new versions of one vendor
- All new versions of one package

However, since roundcube migrated to packagist, I'd like to follow all new versions of [roundcube plugins](https://packagist.org/?type=roundcube-plugin). I tried to use the URL query like https://packagist.org/feeds/releases.rss?type=roundcube-plugin but it doesn't work.

Is this a hidden setting? Or is a thing that need to be implemented?

Start with the releases.rss feed endpoint and compare its current supported filters with the requested type=roundcube-plugin query. Done means the RSS and Atom feeds can return all new versions matching the roundcube-plugin package type, with the existing package and vendor filters still working.

Written by the indexing model from the issue text.