Expand SQL coverage to pushdown all multi-shard queries with colocated tables and subselects

Citus currently provides full SQL coverage for the multi-tenant use case.

For the operational analytics use case, when the user creates multiple colocated tables and has complex subselects, we provide them with rules in how they need to write their queries. This issue tracks expanding our SQL coverage to include all SQL queries that touch colocated tables and subselects.

We're tracking example customer queries in two internal documents (Raw Queries: Expanding SQL Coverage, Summary: Expanding SQL Coverage). I'm opening this issue to track our progress on this task.