[FEATURE] Backup encryption

[runningman84]

Is your feature request related to a problem? Please describe (👍 if you like this request)

It is great that longhorn supports encrypted volumes. In some deployments you want to have the full system encrypted, in talos this is possible using secureboot/tpm:
https://www.talos.dev/v1.6/talos-guides/install/bare-metal-platforms/secureboot/#disk-encryption-with-tpm

In this case encrypting the longhorn volume on top of an already encrypted volumes does not make much sense.

Therefore it would be great to also have an option to only encrypt backups to s3 using some kind of encryption/decryption.

This would reduce the constant performance overhead and only consume cpu cycles in case of actual backup and restore operations.

Describe the solution you'd like

Have the option to encrypt/decrypt s3 backups using some kind of client side encryption like this https://docs.aws.amazon.com/AmazonS3/latest/userguide/UsingClientSideEncryption.html

Describe alternatives you've considered

Have the linux system volumes unencrypted. This leads to a danger of theft.

Additional context

[derekbit]

We already have a ticket #5220 for the feature.