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* feat(backends/s3): add warmup support before repacks and restores This commit introduces basic support for transitioning pack files stored in cold storage to hot storage on S3 and S3-compatible providers. To prevent unexpected behavior for existing users, the feature is gated behind new flags: - `s3.enable-restore`: opt-in flag (defaults to false) - `s3.restore-days`: number of days for the restored objects to remain in hot storage (defaults to `7`) - `s3.restore-timeout`: maximum time to wait for a single restoration (default to `1 day`) - `s3.restore-tier`: retrieval tier at which the restore will be processed. (default to `Standard`) As restoration times can be lengthy, this implementation preemptively restores selected packs to prevent incessant restore-delays during downloads. This is slightly sub-optimal as we could process packs out-of-order (as soon as they're transitioned), but this would really add too much complexity for a marginal gain in speed. To maintain simplicity and prevent resources exhautions with lots of packs, no new concurrency mechanisms or goroutines were added. This just hooks gracefully into the existing routines. **Limitations:** - Tests against the backend were not written due to the lack of cold storage class support in MinIO. Testing was done manually on Scaleway's S3-compatible object storage. If necessary, we could explore testing with LocalStack or mocks, though this requires further discussion. - Currently, this feature only warms up before restores and repacks (prune/copy), as those are the two main use-cases I came across. Support for other commands may be added in future iterations, as long as affected packs can be calculated in advance. - The feature is gated behind a new alpha `s3-restore` feature flag to make it explicit that the feature is still wet behind the ears. - There is no explicit user notification for ongoing pack restorations. While I think it is not necessary because of the opt-in flag, showing some notice may improve usability (but would probably require major refactoring in the progress bar which I didn't want to start). Another possibility would be to add a flag to send restores requests and fail early. See https://github.com/restic/restic/issues/3202 * ui: warn user when files are warming up from cold storage * refactor: remove the PacksWarmer struct It's easier to handle multiple handles in the backend directly, and it may open the door to reducing the number of requests made to the backend in the future.