R1:c84b9d408cb5

Add `bin/almanac register` to associate a host with an Almanac device and trust it

Summary:
Ref T2783. This is basically a more refined version of D10400, which churned a bit on things like SSH key storage, the actual way the signing protocol shook out, etc.

- When Phabricator tries to make an intra-cluster service call as the omnipotent user, sign it with the host's device key.
- Add `bin/almanac register` to say "this host is X device, identified by private key Y". This stores the keypair locally…