Application-level TEA redirects

In the meeting we discussed using the PURL `repository_url` qualifier to locate a TEA server. The downside: this assumes each package repository runs a TEA service for millions of packages, which is unlikely.

As an alternative we could allow allow TEA endpoints (especially `productReleases`) to include **redirects** to external TEA servers, for example by allowing special **link objects** in the response alongside normal `productRelease` objects.

This would let package repositories support TEA at minimal cost by simply redirecting requests to the TEA servers operated by individual package owners.