Add dimensions to RoomFurniturePlacement

[michaelthatsit]

This will help with Auto layout. My current plan is to pass the planes and layout data and have the system use that data, but I'd also need dimension info and don't want to pull the whole asset for something we've precomputed.

[a-type]

Requesting furniture data from the API by ID shouldn't be particularly costly. If this is a convenience or speed issue, we can create a custom endpoint which embeds this data into the full Room data.