[persist] Track whether or not our JSON contains a map

[bkirwi]

Motivation

materialize=> select '7'::jsonb::numeric;
 numeric 
---------
       7
(1 row)

materialize=> select '{}'::jsonb::numeric;
ERROR:  cannot cast jsonb object to type numeric

If we want to be able to push down casts on jsonb columns, it's important that we know when the column is all of a single type. However, currently totally empty jsonb objects -- {} -- don't show up in stats at all. This is a minimal change that captures them in stats in a backwards - compatible way.

Tips for reviewer

This is not how I'd do things from scratch, but it seems like the smallest change that captures the information we need. I left a TODO to revisit this code when we're cleaning up / getting ready to use this for real.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes: