feat: add parallel schema migration helpers to migrate module

Added helper functions for safe 0.14.6 → 2.0 migration using parallel schemas:

New functions in datajoint.migrate:
- create_parallel_schema() - Create _v20 schema copy for testing
- copy_table_data() - Copy data from production to test schema
- compare_query_results() - Validate results match between schemas
- backup_schema() - Create full schema backup before cutover
- restore_schema() - Restore from backup if needed
- verify_schema_v20() - Check if schema is 2.0 compatible

These functions support the parallel schema migration approach which:
- Keeps production untouched during testing
- Allows unlimited practice runs
- Enables side-by-side validation
- Provides easy rollback (just drop _v20 schemas)

See: datajoint-docs/src/how-to/migrate-to-v20.md

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

Author: dimitri-yatsenko