Reconcile data file / sst files between deltacat and rivulet

[mcember]

Acceptance Criteria

• Are we writing data files how deltacat expects? If not, make a storage interface for writing data files and move rivulet memtable_dataset_writer to use that
• In PR 464, we re-use the data_files field in a manifest to point to SST files. Should we instead make an SST_files field or otherwise extend Delta/Manifest to support SST files?
• Should we officially add "level" to delta spec or just keep it as a key that only rivulet understands?