R1:5e0f218fe480

Allow device SSH keys to be trusted

Summary:
Ref T6240. Some discussion in that task. In instance/cluster environments, daemons need to make Conduit calls that bypass policy checks.

We can't just let anyone add SSH keys with this capability to the web directly, because then an adminstrator could just add a key they own and start signing requests with it, bypassing policy checks.

Add a `bin/almanac trust-key --id <x>` workflow for trusting keys. Only trusted keys can sign requests.

Test Plan…