[lib] Introduce encrypted media types

Summary:
Linear task: ENG-386

Added types encrypted_photo and encrypted_video. The only difference is that instead of uri they contain holder and encryptionKey pair.

Added invariant in several places across codebase to indicate places that will be modified in subsequent diffs and to supress Flow errors

Test Plan: Flow doesn't complain. Web and Native don't throw, it is still possible to send unencrypted media messages

Reviewers: atul, ashoat, marcin

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7168