Consider upstreaming CTE support

Background

https://github.com/oxidecomputer/omicron/pull/1733 introduced some utilities to build CTEs:

nexus/src/db/subquery.rs - Adds the definition of traits like Subquery, as well as a CteBuilder to help building arbitrary CTEs.
nexus/db-macros/src/lib.rs - Adds macro to derive Subquery (identifying that a query can be used in a CTE arm)
nexus/db-macros/src/subquery.rs - Implementation of Subquery macro
nexus/src/db/alias.rs - Defines expression which allows field aliasing (FOO as BAR in SELECT/RETURNING queries)
nexus/db-model/src/queries - Uses the table! macro to describe the columns of subqueries. This generates more code than we actually want!

Goal

It would be nice to simplify the "known hacky" pieces of this -- e.g. re-using the table! macro for subqueries -- and to upstream this support into Diesel, to ensure that we don't get broken by future Diesel revisions.

Start with the CTE utilities in nexus/src/db/subquery.rs and the related macro files nexus/db-macros/src/lib.rs and nexus/db-macros/src/subquery.rs, then review nexus/src/db/alias.rs and the queries under nexus/db-model/src/queries. Compare the existing table! use for subqueries with the desired Diesel upstream support; done means defining a concrete upstreamable design for these pieces.