[identity] Verify device list signatures

Summary:
Resolves ENG-7375

Added device list signature verification to Identity Service, when calling the UpdateDeviceList RPC with a signed payload.

Depends on D11957

Test Plan:
Called UpdateDeviceList RPC:

• No signatures pass
• Valid curPrimarySignature pass, invalid fail
• lastPrimarySignature is verified when primary device ID has changed

Reviewers: kamil, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11959