Introduce layered storage architecture with content core type

Three-layer architecture:
1. MySQL types: longblob, varchar, etc.
2. Core DataJoint types: object, content (and @store variants)
3. AttributeTypes: <djblob>, <xblob>, <attach>, <xattach>

New core type `content` for content-addressed storage:
- Accepts bytes, returns bytes
- Handles hashing, deduplication, and GC registration
- AttributeTypes like <xblob> build serialization on top

Naming convention:
- <djblob> = internal serialized (database)
- <xblob> = external serialized (content-addressed)
- <attach> = internal file
- <xattach> = external file

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

Author: claude