[identity] Add function to verify initial device list

Summary:
Added a function that verifies initial device list:

• Verifies that the payload has only curPrimarySignature and the signature is valid
• Verifies that the device list contains only one device and it is the primary device ID
• It does not verify the timestamp - this is done automatically by transaction code used for storing device lists in DDB

Depends on D12067

Test Plan: Tested later in the stack when used

Reviewers: kamil, will, varun

Reviewed By: kamil, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12089