Return osystems and distro_series requiring keys from the general.osinfo websocket API

[Kit Randel]

To implement the license key interface, we need osinfo (websockets.general.osinfo) to return new keys `osystems_requiring_keys` and `releases_requiring_keys` displaying only osystems and distro_series that require license keys.

[Kit Randel (blr)]

I've managed to implement what we need using the existing API. Although it might be a bit cleaner to return explicit sets of osystems and distro_series that require license keys, it isn't pressing, so feel free to deprioritise the API change request.