[Identity] Handle key payload

Summary:
Allow for the key payload to be serialized and deserialized.

This is needed later for verifying that the payload is correctly
structured and a device is also identified by the identity public key.

Test Plan:

cd services/identity
cargo test

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7298