R1:67084a69533c

Support AES256 at-rest encryption in Files

Summary:
Ref T11140. This makes encryption actually work:

- Provide a new configuation option, `keyring`, for specifying encryption keys.
- One key may be marked as `default`. This activates AES256 encryption for Files.
- Add `bin/files generate-key`. This is helps when generating valid encryption keys.
- Add `bin/files encode`. This changes the storage encoding of a file, and helps test encodings and migrate existing data.
- Add `bin/files cycle`.