Evaluate removing duplicated code from Citus by proposing changes to PostgreSQL

We needed to make several changes as part of the extensification work that duplicated logic from PostgreSQL. These include ruleutils.c, Citus node types and functions, and list functions for tasks (and custom scans?).

If we could remove at least some of these duplications, that would be nice. Evaluate if we have simple change proposals we can make to PostgreSQL core that will help us remove some of the duplicate work.