u3d/available: fetch versions data from a central repository, speeding up identification of available releases

[lacostej]

Not everyone might want to use u3d, but having the list of licenses might be interesting. We could make a cron job that keeps this information available on a service.

Also answer https://support.unity3d.com/hc/en-us/articles/211013503-How-can-I-have-a-fully-automated-build-process-that-can-fetch-and-install-any-version-of-Unity-into-our-build-environment-

The complete idea is the following

• make a service that creates an online cache

U3dCore::Helper.operating_systems.each do |os|
	U3d::Cache.new(force_os: os, force_refresh: true)
end

• cache this online every now and then
• update u3d to fetch from that service first.

Advantages:

• we can provide this information to humans (see support question above)
• we can provide this information to other programs
• we can provide this information to u3d itsel: if the u3d available code breaks again, older versions don't need to be updated. This would also make available much faster.
• if that server is unavailable, fallback on fetching information yourself
• people should be able to disable that feature, but it could be on by default