SoftDeletesRetentionMergePolicy class not final

### Description

I wonder if there are any possilbe that the class `SoftDeletesRetentionMergePolicy` can be not a final class, so that the user can do something such as cache, accroding to their `retentionQuerySupplier` by extend this class

Start by locating the SoftDeletesRetentionMergePolicy class and reviewing how retentionQuerySupplier is used. Check related tests and API constraints before deciding whether subclassing and caching are supported; done should clearly define the intended extension behavior and preserve existing retention-policy behavior.

Written by the indexing model from the issue text.