[native] Add function to decrypt media file

Summary:
This diff adds function that opens and decrypts given encrypted file. Decrypted content can either be saved to a file or returned as a data URI.

Depends on D7223

Test Plan: This function is tested in subsequent diffs where it is used.

Reviewers: atul, marcin, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7224