[identity] Get and verify initial device list from registration requests

Summary:
Added some helper traits and implemented them for all request types that are registration-related (have initialDeviceList).
This is similiar to the existing DeviceKeyUploadData and DeviceKeyUploadActions which support common functionality for all request types that have device keys payload.

The trait provides a single function for extracting and verifying the initial device list. The construct_user_registration_info() calls it. The device list is then stored for further processing (next diff).

Depends on D12089

Test Plan: Tested in the next diff when all pieces are joined together.

Reviewers: kamil, will, varun

Reviewed By: will, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12090