Create encrypted backup files

To provide secure on-rest of backup files, the backup agents or fdbserver (i.e., the backup worker role) should support the option of creating encrypted files. This requires a way for backup agents or fdbserver to securely obtain encryption keys, which needs more thoughts. At a minimum, we should have some APIs for encrypting the backup files while generating them.

Start by reviewing the backup agents and fdbserver backup worker role described in the issue, then clarify how encryption keys should be obtained securely. The work is complete when the scope and APIs for encrypting backup files during generation are agreed and implemented.

Written by the indexing model from the issue text.