[identity] replace deviceID concept with signingPublicKey

Summary:
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

Test Plan: 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

Reviewers: jon, tomek, bartek

Reviewed By: jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6840