the ed25519 public key that users use to sign messages will replace our device ID concept. we'll use this key to map to all the other device-specific session initialization info (read: other keys)

modified our terraform schema and identity client in the keyserver to match this new API design

Depends on D6839

successfully built the identity service. then, i tried calling the registerUser client method from the keyserver and checked that the response and DDB matched the input from my calls