[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