Depends on D3848.
I got frustrated with all the code that was required to define a basic lakehouse. The goal here is to make defining a storage a little more elegant. Alex suggested a similar approach in a recent review.
Independently, it now matches in-memory types to storage policies via "isinstance", not exact match. This makes it easy to, for example, create a mock policy that applies to all types, instead of painstakingly creating a policy per type.