Implement storage of a host ID and a public key for authorizing Conduit between servers

Summary:
Ref T4209.  This creates storage for public keys against authorized hosts, such that servers can be authorized to make Conduit calls as the omnipotent user.

Servers are registered into this system by running the following command once:

```
bin/almanac register
```

NOTE: This doesn't implement authorization between servers, just the storage of public keys.

Placing this against Almanac seemed like the most sensible place, since I'm imagining in future that the `register` command will accept more information (like the hostname of the server so it can be found in the service directory).

Test Plan: Ran `bin/almanac register` and saw the host (and public key information) appear in the database.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4209

Differential Revision: https://secure.phabricator.com/D10400