[identity] Add function to batch-get platform details

Summary:
Part of GetDeviceListsForUsers RPC rewrite. Details in ENG-8843.

Added a function that fetches PlatformDetails for multiple devices using BatchGet

Depends on D12888

Test Plan: Detailed test plan in ENG-8843.

Reviewers: varun, will, kamil

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12889