Serialize sstable keyspace and table metadata on StatsMetadata component

Currently Cassandra relies on the parent directory structure to figure out the keyspace/table of a given sstable: $data_dir/$keyspace/$tablename-$tableId/$sstable.

Offline tools require sstables to be placed in the well-defined directory structure above to correctly parse keyspace/table information.

We can improve this by serializing keyspace/table metadata (such as keyspace/table names and table id) into the StatsMetadata component, so offline tools can read sstables from any parent directory.