[identity] more helpers for identity service

Summary:
From the device_info HashMap, we need to construct an InboundKeyInfo struct. These are some helper functions to do that.

Depends on D8761

Test Plan: Tested in subsequent diff when we actually call the helpers

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8765