Allow us to mark parts as constant so they never unload with streaming!

[OrbitalOwen]

We attempted to work around this by moving models we wanted to make constant into the ReplicatedStorage and then had the client parent them back into the Workspace. Having learned this does nothing to stop parts from unloading, we are scrambling to figure out how to fix this quickly, and all existing solutions are very painful.

Clone them before parenting to Workspace instead. See: Streaming Prefetch API and Improvements to Part Handling