For tables using tablet based replication strategies, the sstables
should be reshaped only within the compaction groups they belong to.
Updated shard_reshaping_compaction_task_impl to group the sstables based
on their compaction groups before reshaping them within the groups.
Fixes#16966
Signed-off-by: Lakshmi Narayanan Sreethar <lakshmi.sreethar@scylladb.com>