[protos] Add PlatformDetails to PeersDeviceListsResponse

Summary:
Extended response for device list multi-fetch RPC to include PlatformDetails for each device.

Unfortunately, proto doesn't allow for map<K_outer, map<K_inner, V>> directly, so needed to introduce intermediate message type.

This message could be nested, but grpc-web codegen generated Flow-unfriendly types.

Depends on D12271

Test Plan: proto compiles, identity compiles, flow for web

Reviewers: ashoat, marcin

Reviewed By: marcin

Subscribers: ashoat, varun, tomek

Differential Revision: https://phab.comm.dev/D12272