Add dynamic field extraction

[yedidyakfir]

Goal

Allow runtime field path resolution

Description

Enable extracting multiple fields at runtime using string-based field path specifications.

Example

values = await model.extract_fields([
    'field_lst[0].field_int',
    'field_lst[1].field_str'
], redis_id)

Acceptance Criteria

• Implement extract_fields method
• Parse string field paths at runtime
• Support array indexing in paths
• Return dict of field values
• Add tests for dynamic extraction
• Document the extraction API