[identity][grpc_clients] Move PlatformMetadata to grpc_clients crate

Summary:
Decided to make PlatformMetadata shared between Identity Service and all clients.
This lets us avoid some code duplication.

Also added some helpers that will be handy in the very next diffs

Depends on D12280, D12274

Test Plan: Identity and all clients compile.

Reviewers: varun, marcin

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12282