[identity] introduce new ping RPC and code version interceptor function

Summary: pretty straightforward... just a no-op RPC that will return an unsupported version status code if the caller's code version is outdated. will set the actual unsupported versions for each platform when we launch.

Test Plan: called the RPC from kreya with metadata to trigger both an OK response and an unsupported version response

Reviewers: bartek, michal, jon

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9479