sql/catalog: remove SchemaKind from SchemaDescriptor

[ajwerner]

Is your feature request related to a problem? Please describe.

Before #65142, we had a concept ResolvedSchema which either had a descriptor or had just a modicum of information. This allowed us to cope with the weirdness of public, virtual, and temporary schemas (more or less). In the above referenced PR, we unified this concept around different implementations of the descriptor interface. However, the behavior of these different kinds of descriptors is not perfectly consistent and they do not provide all of the features of a normal schema descriptor. Namely:

• ParentID is not always set
• Privileges are not always set

The reason for this is that the objects are singletons today. It seems scary to allocate new objects every time. It feels like a big disruption to #65142 to add it to that already non-trivial and blocking PR.

Describe the solution you'd like

We should store objects for these things from a node-level cache provided to the collection.

Epic: CRDB-2454

Jira issue: CRDB-7804

[postamar]

In 22.2 there won't be anything blocking us from doing this anymore.

[postamar]

I haven't found the lack of ParentID to be particularly annoying but this is probably worth doing for privileges. It shouldn't be particularly hard.