Feature: add "marked deleted" rows count to the table stats API

### SDK

Rust

### Description

add to `TableStatistics` the summary of over all fragments of rows marked for deletion.
this could be used as an indication that `table.optimize()` should be called

Start at the TableStatistics API and trace how table fragments contribute to its existing summaries. Compare the new marked-for-deletion count with the behavior of table.optimize(), then confirm that the API reports the aggregate across all fragments and can indicate when optimization may be useful.

Written by the indexing model from the issue text.