Create Convenient Way to Iterate Over Casted Asset Packs

[zicklag]

Right now it's annoying to loop through all the asset packs, get their root asset handle, cast it to the appropriate type, and then get the asset from the asset server.

We should make a convenient way to access all of the pack assets that cast to a certain type, maybe in a system parameter like we already do with Root<GameMeta>, except it'd be Packs<PackMeta>.