[web] factor out creation of DeviceKeyUpload

Summary:
we will need to create an object of this type for wallet login too, so moving the common code to a helper

Depends on D10959

Test Plan: successfully logged in to identity service on web, console logged DeviceKeyUpload to make sure it matched the contents in DDB

Reviewers: ashoat, tomek, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D10960