[Enhancement] Simplify Import Paths by truncating py4j

[chenghuichen]

Currently, users need to import core classes like Catalog and Table from the implementation-specific module py4j:

from pypaimon.py4j import Catalog
from pypaimon.py4j import Table

This exposes internal technical details and creates unnecessary coupling between user code and the implementation mechanism.

I propose to expose these core classes at the package root level by re-exporting them in init.py, allowing users to import directly from the main package:

from pypaimon import Catalog
from pypaimon import Table

Benefits:

• Cleaner and more intuitive API - users don't need to know about the underlying Py4J implementation
• More flexibility for future refactoring - we can change the underlying implementation without breaking user code
• Follows Python best practices of providing a clean public API

This change maintains backward compatibility while providing a better developer experience.