[identity] Add device list timestamp verification

Summary:

• Updated transact_update_devicelist to accept input timestamp, instead of always generating new. It's still optional for Identity-managed device list updates
• Added timestamp verification criteria according to the whitepaper

Test Plan:

• Manually called the UpdateDeviceListRPC with valid / invalid timestamps
• Updated integration test (previously failing due to dummy timestamp)
• Added unit tests

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11685