[keyserver] Create an endpoint returning primary invite links

Summary: This endpoint will be used to display exiting link. In near future we will need an endpoint that returns all the links - it will reuse most of the logic from this endpoint.

Test Plan: Call the endpoint and check if all the primary links were returned.

Reviewers: bartek, kamil, inka

Reviewed By: kamil

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7726