Add new scylla_metadata_type::SSTableOrigin. Store and retrive a sstring to the scylla metadata component. Pass sstable_writer_config::origin from the mx sstable writer and ignore it in the k_l writer. Add unit test to verify the sstable_origin extension using both empty and a random string. Signed-off-by: Benny Halevy <bhalevy@scylladb.com>