[keyserver] Process and store encrypted media

Summary:
Part of ENG-3393

Encrypted media can no longer be processed on keyserver. Modified validateAndConvert - added a simple validation based on provided MIME type - it must be provided in order to detect if we have image or video. Then the content is passed through.

Then stored the encryption key in extras column of the uploads table.

Depends on D7166

Test Plan: Modified the uploadMultimedia call in input-state-container to pass some encryptionKey. Uploaded some media. Ensured the encryptionKey is added to the uploads table

Reviewers: atul, marcin, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7167