feat: Add external storage and filepath migration functions

Add migrate_external() and migrate_filepath() to datajoint.migrate module
for safe migration of 0.x external storage columns to 2.0 JSON format.

Migration strategy:
1. Add new <column>_v2 columns with JSON type
2. Copy and convert data from old columns
3. User verifies data accessible via DataJoint 2.0
4. Finalize: rename columns (old → _v1, new → original)

This allows 0.x and 2.0 to coexist during migration and provides
rollback capability if issues are discovered.

Functions:
- migrate_external(schema, dry_run=True, finalize=False)
- migrate_filepath(schema, dry_run=True, finalize=False)
- _find_external_columns(schema) - detect 0.x external columns
- _find_filepath_columns(schema) - detect 0.x filepath columns

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko