Enhancement: Add support for `credential_process` to S3 backend

We have updated the library used for the S3 backend. This adds support for the
`credential_process` option in the aws credential configuration.

https://github.com/restic/restic/issues/3029
https://github.com/restic/restic/issues/4034
https://github.com/restic/restic/pull/4025
