feat: expose catalog metadata api for pypaimon_rust

[SML0127]

Purpose

Linked issue: close #284

Brief change log

• added list_databases, list_tables, and get_table methods to existing PaimonCatalog.
• introduced new PyO3 wrapper classes for metadata inspection: Table, TableSchema, and DataField.

Tests

API and Format

• Python-only API additions. No changes to Rust public APIs or storage formats.

Documentation

[leaves12138]

Thanks for the PR. The runtime binding implementation looks generally good, and the existing CI is green. I also verified locally that cargo test -q -p pypaimon_rust --no-default-features passes after the first build.

One blocking issue: this package ships py.typed and bindings/python/python/pypaimon_rust/datafusion.pyi, but the stub file still only exposes PaimonCatalog.__init__ and __datafusion_catalog_provider__. The new public APIs added by this PR (list_databases, list_tables, get_table, plus the new Table, TableSchema, and DataField classes and their methods) are missing from the type stubs.

That means dynamic Python calls work, but type-aware users and IDEs will still see the old API. Please update bindings/python/python/pypaimon_rust/datafusion.pyi to match the new exported Python surface. It would also be helpful to cover this with a small stub/import check if the project already has a place for that.

Also, GitHub currently reports the PR as not mergeable (mergeable_state=dirty), so it will need a rebase/update before merge.

[SML0127]

@leaves12138 Thanks for the comment. I've updated and rebbased the branch.