Add "Something" to expose for Asset mod compatibility

[legoandmars]

Things like LethalSDK and LethalLevelLoader create custom content that has to be synced, but don't require plugins
It might be good to have them register into LobbyCompatibility somehow

Maybe post-release

[Xilophor]

Looks like this may be able to be resolved with #37.

[Xilophor]

From IAmBatby:

in a perfect world ig it would be nice if i could do something like

LobbyCompatability.RegisterComparableDataSet<T[] dataSet>(); or something

then for example it would be generic enough for to use this to check if client and host both have the same moons and dungeons loaded and their set to be registered in their configs (cuz in LLL you can potentially have content installed but not enabled)