sstables::types: Add optional extensions attribute to scylla metadata
sstables::types: Add optional extensions attribute to scylla metadata

Allowing storing key:value pairs. 
duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla