Encryption for backup and restore

[millasml]

Description

Supports suppling an encryption key file path for backups and restores.

Type of change

• New feature (non-breaking change which adds functionality)

Discussion

Are there any design details that you would like to discuss further?

Testing

Please describe the tests that you ran to verify your changes. Unit tests? Manual testing?

Do we need to perform additional testing once this is merged, or perform in a larger testing environment?

Documentation

Did you update relevant documentation within this repository?

If this change is adding new functionality, do we need to describe it in our user manual?

If this change is adding or removing subreconcilers, have we updated the core technical design doc to reflect that?

If this change is adding new safety checks or new potential failure modes, have we documented and how to debug potential issues?

Follow-up

Are there any follow-up issues that we should pursue in the future?

Does this introduce new defaults that we should re-evaluate in the future?