Update storage types spec with OAS integration approach

- Clarify OAS (object type) as distinct system
- Propose storing blob@store/attach@store in OAS _external/ folder
- Content-addressed deduplication via hash stored in varchar(64)
- Propose <ref@store> to replace filepath@store
- Add open questions and implementation phases

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude