R1:5dccc14bbf46

Modularize generation of supplemental login messages

Summary:
Ref T9346. This mostly allows us to give users additional advice based on which instance they are trying to log in to in the Phacility cluster.

It's also slightly more flexible than `auth.login-message` was, and maybe we'll add some more hooks here eventually.

This feels like it's a sidegrade in complexity rather than really an improvement, but not too terrible.

Test Plan:
- Wrote the custom handler in T9346 to replicate old config…