[python] Add DataFusion extra for merge conditions

[XiaoHongbo-Hope]

Purpose

merge_into condition expressions use the Python DataFusion package but currently direct users to pypaimon[sql], which also installs pypaimon-rust. This makes the condition dependency unnecessarily coupled to the full SQL stack.

Changes

• Add pypaimon[datafusion] with only datafusion>=54,<55 on Python 3.10+.
• Keep pypaimon[sql] unchanged for the full Rust and DataFusion SQL stack.
• Point merge condition errors and documentation to the new extra.
• Fail with an explicit Python upgrade message on Python 3.6-3.9.
• Quote the installation command for shells such as zsh.

Validation

• Merge condition tests: 13 passed, 36 skipped, 109 deselected.
• Missing-dependency and unsupported-Python regression tests passed.
• Generated package metadata contains the new datafusion extra while sql retains both dependencies.
• flake8 and git diff --check passed.

[JingsongLi]

+1