[keyserver] Support encrypted media in create_multimedia_message

Summary: A few more changes needed for create_multimedia_message endpoint to properly handle encrypted multimedia. This was not needed for native, but is needed for web where message is created from pre-uploaded media.

Test Plan: Encrypted multimedia messages can be created from uploads having encryption key in upload extras.

Reviewers: atul, marcin

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7291