[native] Calculate SHA256 of encrypted blob

Summary:
We need the blob hash in order to upload the encrypted blob to Blob service.
According to whitepaper, we use SHA256 to calculate the hash of encrypted blob.

Exposed the calculated SHA256 within encryption result payload.

Test Plan: Added console.log and sent encrypted video message. Verified that both video and thumbnail have blobHash field.

Reviewers: atul, marcin

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7647