Enhancement: Add warmup support on S3 backend before repacks and restores

Introduce S3 backend options for transitioning pack files from cold to hot
storage on S3 and S3-compatible providers. Note: only works before repacks
(prune/copy) and restore for now, and gated behind a new "s3-restore" feature
flag.

https://github.com/restic/restic/pull/5173
https://github.com/restic/restic/issues/3202
https://github.com/restic/restic/issues/2504
