[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