[lib][keyserver] Expose upload_media_metadata endpoint
Summary:
This diff exposes the upload_media_metadata endpoint.
Depends on D7619
Test Plan:
Created useServerCall hook and called this endpoint. Verified that a new record is added to the uploads table.
The endpoint response format is the same as the upload_multimedia endpoint.
Reviewers: ashoat, atul, marcin, inka
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D7620