[native] Add function to encrypt media during media mission

Summary:
This diff introduces a function that will be called during media mission - it takes media processed by processMedia() (most of media mission) and processes it further by applying encryption.

Depends on D7233

Test Plan: This function is tested in the next diff

Reviewers: atul, marcin

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7234