Load new ego public key on demand

[KevinFHartmann]

Create a setting, endpoint, signal handler, or other way to tell program-service to get a new public key from Ego.

This ensures that:

a) old tokens signed with a compromised key can be immediately invalidated (they won't pass authorization against the new key) (currently, we use the same public key until the service is re-started)

b) we can switch to a new ego server without bringing Program Service down and back up.