Bind std::function variables as named Python functions

[ProfFan]

Global std::function variables currently become anonymous Python callables through module.attr(...) assignment. GTSAM’s DefaultKeyFormatter, for example, works at runtime but lacks function metadata, causing pybind11-stubgen to omit it.

Generate a C++ helper that selects module.def(...) for std::function values and attribute assignment for ordinary variables. C++ overload resolution handles aliases such as KeyFormatter without application-specific rules. Preserve empty functions as None and the direct C++ callback path for stored function pointers.

Updates documentation and expected generated output. MATLAB generation is unaffected.

Validation:

• Wrap: 132 tests passed; 2 annotation tests deselected to avoid invoking a compiler.
• Full replacement in GTSAM: all bindings regenerated; 541 stable tests passed, 6 skipped; 6 unstable tests passed.
• Manually verified callable metadata, C++ callbacks, and stub declarations and exports. Stubgen reported no errors.
• No new compiler-dependent tests or stubgen dependency.

Related: borglab/gtsam#2789.

[dellaert]

Mostly trusting you :-)