Add unique constraint and many-to-many relationship metadata to the
generated OpenAPI spec so clients can render them.
- Store unique constraints on Table as tableUniqueCols (mirroring
tablePKCols) instead of denormalizing them onto each Column.
- Compute unique constraints via a per-table tbl_unique_cols CTE in
tablesSqlQuery.
- Annotate unique columns and composite unique constraints in property
descriptions, and emit m2m markers in table descriptions.
jimmy