[lib] Allow calling link verification from clients
Summary:
This diff adds some code that allows calling link verification endpoint.
Depends on D7521
Test Plan: Tested in combination with next diffs - every call from link handler (introduced later) verifies this code.
Reviewers: bartek, kamil, inka
Reviewed By: bartek
Subscribers: ashoat, atul
Differential Revision: https://phab.comm.dev/D7522