Constraints rpc transition (list, delete & add)

[Anish9901]

Fixes #3594

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[mathemancer]

LGTM. Nice work to both @seancolsen and @Anish9901 on this one!

[seancolsen]

We exchanged a few messages in Matrix regarding the new pattern I introduced in this PR of RETURN TABLE. I just noticed an additional point in favor of this new pattern that I'd like to highlight here...

When OID values are serialized to JSON via psycopg (as with RETURN TABLE), those OID values end up as JSON numbers. That's good! In contrast, serializing OIDs to JSON within PostgreSQL requires us to manually cast them to bigint, making the RETURN jsonb pattern more prone to developer error, as in this bug.