Add support for Bifroest's bridge descriptor tarballs

The current bridge authority Tonga will be shut down in a few days. The new bridge authority Bifroest is up and running for a few days now and will take over. See #19690 (moved) for details about the transition.

We'll have to handle Bifroest's bridge descriptor taballs, which we can't do right now. I'll post a branch in a second.