Implement configuration extension support

[timsaucer]

Which issue does this PR close?

Closes#1390

Rationale for this change

With the upstream repository now supporting FFI configuration options, we should support these in datafusion-python.

What changes are included in this PR?

Adds methods to add config extensions to SessionConfig

Are there any user-facing changes?

New addition. Existing methods are not impacted.

[Copilot]

Pull request overview

Adds support for DataFusion configuration extensions so Python SessionConfig can be augmented with upstream/FFI-provided extension options (and demonstrates usage via the FFI example crate).

Changes:

• Add SessionConfig.with_extension(...) to pass config extensions through the Python API.
• Implement PySessionConfig.with_extension(...) in Rust, extracting/merging FFI_ExtensionOptions from a PyCapsule.
• Extend the FFI example crate with a MyConfig extension and a Python test demonstrating SHOW/SET against the extension.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
python/datafusion/context.py	Exposes SessionConfig.with_extension(...) on the public Python wrapper.
crates/core/src/context.rs	Adds Rust binding logic to extract/merge extension options from a Python object/capsule.
crates/core/src/dataset_exec.rs	Minor type annotation change for schema (clarifies type inference).
examples/datafusion-ffi-example/src/lib.rs	Registers the new config module and exports MyConfig to Python.
examples/datafusion-ffi-example/src/config.rs	Implements a sample ConfigExtension/ExtensionOptions plus a capsule exporter method.
examples/datafusion-ffi-example/python/tests/_test_config.py	Adds an example test covering config extension usage via SHOW/SET.

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