Add afind keywords support (field='text' syntax)

[yedidyakfir]

Goal

Support keyword-based search with afind

Description

Enable keyword-based search using field='value' syntax for simpler queries.

Example

users = await User.afind(status='active', role='admin')

Acceptance Criteria

• Support **kwargs in afind for field=value syntax
• Convert keyword arguments to filter expressions
• Combine with existing filter expressions
• Add tests for keyword-based search
• Document the new syntax