Add DEP-06: Immutable ETCD Backups (#884)

* Add DEP-06: Immutable ETCD Backups

Co-authored-by: Saketh Kalaga <51327242+renormalize@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Saketh Kalaga <51327242+renormalize@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Madhav Bhargava <madhav.bhargava@sap.com>

* Apply suggestions from code review

Co-authored-by: Ishan Tyagi <42602577+ishan16696@users.noreply.github.com>
Co-authored-by: Shreyas Rao <42259948+shreyas-s-rao@users.noreply.github.com>

* Address review comments for immutable etcd backups proposal

- Added example YAML configuration for `etcd.spec.backup.store`.
- Included note on credential rotation management.
- Enhanced hibernation flow with traffic cut-off step.
- Detailed lifecycle of `ExtendFullSnapshotImmutabilityTask`.
- Added section on disabling immutability.
- Included note on AWS S3 bucket support for ignoring snapshots during restoration.

* Addressed review comments

- Add new CLI command `extend-snapshot-immutability` to replace `reupload-snapshot`.
- Update documentation to reflect changes and provide detailed steps for handling hibernated clusters.
- Remove redundant garbage collection configuration from `ExtendFullSnapshotImmutabilityTaskConfig`.

* Address review comments from Shreyas

- Clarify the mechanism to signal hibernation intent in the non-goals section.
- Document the possibility of using provider APIs for server-side object copy to reduce network costs and latency.

* Address review comments from Anvesh

* Address review comments:
- Add link to `EtcdCopyBackupTask` sample YAML
- Add DEP-06 to the README.md

* Apply suggestions from code review

Co-authored-by: Ishan Tyagi <42602577+ishan16696@users.noreply.github.com>

* Update docs/proposals/06-immutable-etcd-backups.md

Co-authored-by: Ishan Tyagi <42602577+ishan16696@users.noreply.github.com>

* Address review comment

- Clarify `--gc-from-timestamp` parameter description

* Update docs/proposals/06-immutable-etcd-backups.md

Co-authored-by: Shreyas Rao <42259948+shreyas-s-rao@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Madhav Bhargava <madhav.bhargava@sap.com>

* Address review comment: Add condition to delete ExtendFullSnapshotImmutabilityTask when immutability configuration is removed from etcd.spec.backup.store

---------

Co-authored-by: Saketh Kalaga <51327242+renormalize@users.noreply.github.com>
Co-authored-by: Madhav Bhargava <madhav.bhargava@sap.com>
Co-authored-by: Ishan Tyagi <42602577+ishan16696@users.noreply.github.com>
Co-authored-by: Shreyas Rao <42259948+shreyas-s-rao@users.noreply.github.com>

docs/README.md

@@ -45,4 +45,5 @@
 * [DEP-2: Snapshot compaction](proposals/02-snapshot-compaction.md)
 * [DEP-3: Scaling up an Etcd cluster](proposals/03-scaling-up-an-etcd-cluster.md)
 * [DEP-4: Etcd Member custom resource](proposals/04-etcd-member-custom-resource.md)
-* [DEP-5: Etcd Operator Tasks](proposals/05-etcd-operator-tasks.md)
+* [DEP-5: Etcd Operator Tasks](proposals/05-etcd-operator-tasks.md)
+* [DEP-6: Immutable etcd Cluster Backups](proposals/06-immutable-etcd-backups.md)