- Added `RetentionExpiry` field to Snapshot struct to store retention expiry time.
- Added `IsDeletable` method to `Snapshot` struct to determine if a snapshot is deletable based on `RetentionExpiry`.
- Introduced `ExcludeSnapshotMetadataKey` constant for marking snapshots to be ignored.
- Modified garbage collection logic to skip immutable snapshots.
- Updated GCS snapstore list function to exclude objects with `x-etcd-snapshot-exclude` metadata.

…a `bool` argument.

…napshots.

…e tagged snapshots' behavior.

* Remove hardcoding of tags in mock GCS tag implementation.

* Rename the `tagI` interface to `tagger`.

* Rename the methods of the `tagger` interface to plural since they
  operate on all tags of the snapshot.

* Logs for skipping GC with immutable objects mention "immutability period"
  and not "retention period" since retention period as a phrase is already
  used for delta snapshot retention.

* Enhance `ExcludeSnapshotMetadataKey` and `List` method comments.

* Eliminate the potential for a flaky test in snapshot_test.go
  where later expiry time is only set to a nanosecond after the
  creation time instead of a microsecond. The test could flake if the
  test gets run within a microsecond.

* Removed one hardcoding of a string from the proposed changed, and
  quite a bit of hardcoding in the existing codebase in snapstore_test.go

* This rename makes the distinction clearer between retention expiry
  which is used for delta snapshots typically, and the immutability
  expiry time of a snapshot.

…pshots.md`