R1:5f29ccaacae0

Add storage for Auth configuration in preparation for moving it into a web interface

Summary:
Ref T1536. Currently, we have about 40 auth-related configuration options. This is already roughly 20% of our config, and we want to add more providers. Additionally, we want to turn some of these auth options into multi-auth options (e.g., allow multiple Phabricator OAuth installs, or, theoretically multiple LDAP servers).

I'm going to move this into a separate "Auth" tool with a minimal CLI (`bin/auth`)…