[lib] add IdentityDeviceListUpdated P2P message type

Summary:
Part of ENG-7837.

This is different than client DeviceListUpdated because:

1. Indicating that this message was sent by Identity.
2. In this case we also want the receiving device to re-send this to all other peers.

Test Plan: Tested in D12378.

Reviewers: bartek, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12375