[identity] Add device list signatures to serde

Summary: Added server-side fields for device list signatures, according to Whitepaper 6.2.3

Test Plan: Unit tests for serialization and deserialization. Existing device logic stays unchanged

Reviewers: kamil, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11777