Scan UuidGenerator implementations without starting them

[mpkorstanje]

The ideal solution would be to have the UuidGenerator return a supplier instance that keeps the state internally.

public interface UuidGenerator {

    Supplier<UUID> supplier();
}

And then have the UuidGeneratorServiceLoader return that supplier only for the selected generator. Fortunately the UuidGenerator is experimental so I don't feel too bad about breaking the contract. But I'd rather avoid doing that on a patch version.

Originally posted by @mpkorstanje in #2931 (comment)