chore: introduce `RemoteResource<T>` interface

[skaldarnar]

Contains

Both games and JREs will be "installed" and managed by the launcher in the future.

I believe that both can have a shared interface for the remote resource, that is, the download URL.

I'm not sure about other properties yet, like a checksum for the file. For managed JREs, the checksum will be statically known. For game releases, we can fetch the checksum separately from remote. Probably should also be a Future<String> to express the async nature, or just a String with some IOException? :thinking:

Also start turning the static utility class DownloadUtils into a class that can be instantiated. This should help long-term with testability of the different manager classes, and make it easier to make the timeouts configurable and testable.

How to test

The changes should be a refactoring from the user's point of view. Ensure that the launcher is working as before.

I've tested this on Windows by downloading a release and starting it.

Outstanding before merging

Nothing.