Implement incremental restore for global indexes

Problem:
- Incremental backup supported indexes but restore did not
- Index data was backed up but not restored

Implementation:
1. Index Discovery and Restore:
   - Added DiscoverAndCreateIndexRestoreOperations() to scan __ydb_backup_meta/indexes
   - Implemented recursive traversal for nested table structures
   - Fixed path matching to avoid false positives (exact match or with '/' separator)
   - Creates restore operations for each index impl table found

2. Schema Version Synchronization:
   - Relies on CDC sync functions already present in main
   - Main's SyncImplTableVersion and SyncIndexEntityVersion handle version sync
   - No additional AlterTableIndex operations needed

3. Testing:
   - Added 5 comprehensive test cases covering various scenarios
   - Tests verify both data integrity and index functionality
   - Direct index impl table verification for stronger assertions

Files Modified:
- schemeshard_impl.h: Function declarations for index discovery
- schemeshard_incremental_restore_scan.cpp: Index discovery and restore logic
- datashard_ut_incremental_backup.cpp: Test cases

This ensures indexes are properly restored and remain queryable.

Author: claude