[web] Add encryptionKey to PendingMultimediaUpload

Summary:
The PendingMultimediaUpload type needs to store information if the media is encrypted along with encryption key

Depends on D7281

Test Plan: Flow check. Picked media is displayed in chat input bar.

Reviewers: atul, marcin

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7292