Encryption with S3 Driver

## Is your feature request related to a problem? Please describe.

I'd like to outsource the user data using an S3 Endpoint.
I don't want to trust the storage provider.

## Describe the solution you'd like

The S3 driver allows to specify an encryption key with which the
data is encrypted before sending it to the data provider.

## Describe alternatives you've considered

File-based encryption keys,
Encryption at higher level.

## Additional context

I don't think it is sensible to add encryption into the ocis driver, because the encryption key has to be on the same machine.