Nested Third-Party Relationships

Third-party relationships used to support exactly one level: a vendor and its direct sub-vendors. Anything past that had no home in the data model.

Third parties can now nest to arbitrary depth. Each third party carries a reference to its parent, and the console, API, and CLI (prb third-party list --level N) all understand levels beyond the first. If your vendor’s subprocessor has its own subprocessor, that relationship is now representable instead of flattened.

This also cleans up how sub-vendor links work generally: they’re just a parent reference on the third party itself now, instead of a separate link/unlink relationship to manage.