Enhancement: Add warmup support for check command on S3 backend

Like the other cold storage features, warmup support for the `check` command
remains experimental and gated behind the "s3-restore" feature flag.

https://github.com/restic/restic/pull/5248
https://github.com/restic/restic/issues/3202
https://github.com/restic/restic/issues/2504
