[keyserver] Compose encrypted media messages

Summary:
Part of ENG-3393

This diff implements composing encrypted messages on keyserver, basing of existence of the encryptionKey field in uploads table.

Depends on D7170
Depends on D7167

Test Plan: This code was actually used to test previous diffs. At this point, multimedia containing encryptionKey field in extras column of uploads table should be shimmed properly

Reviewers: rohan, atul, marcin

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7171