[identity] Fetch platform details along with device lists

Summary:
Following D12272, updated device list multifetch RPC handler to additionally include PlatformDetails for each device.

Depends on D12272

Test Plan:
Repeated test plan for previous diffs in the stack (login devices).
Then called the RPC. Verify that response contains platform details for each device.

Reviewers: varun, marcin, will

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12273