API: Add the implementation for index_summary_off_heap_memory
API: Add the implementation for index_summary_off_heap_memory

This adds the implementation for the index_summary_off_heap_memory for a
single column family and for all of them.

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>

raphaelsc/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla