sstable: Add get_summary function
sstable: Add get_summary function

The get_summary method returns a const reference to the summary object.

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

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