Add option to sstablepartitions for aggregating stats

The sstablepartitions offline tool added by CASSANDRA-8720 processes sstables one by one. If a partition is spread across multiple sstables, the tool will separately report the metrics of the partition fragment on each sstable. For example, if we are looking for partitions larger than 100MiB, and a partition has a 50MiB fragment in one table and 50MiB in another table, the partition won't be found.

It would be useful to add a kind of --aggregate option to sstablepartitions so it aggregates the results of all the analyzed sstables that belong to the same table.