[lib] Add client side boilerplate for the new endpoint

Summary:
This diff adds some code that allows calling link fetch endpoint.

Depends on D7727

Test Plan: Tested in combination with next diffs.

Reviewers: bartek, kamil, inka

Reviewed By: bartek, kamil

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7728