Enhancement: Add experimental S3 cold storage support

Introduce S3 backend options for transitioning pack files from cold to hot storage 
on S3 and S3-compatible providers. Note: this only works for the `prune`, `copy` 
and `restore` commands for now.

This experimental feature is gated behind the "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