Add bindings for `LogicalPlan::Extension`

[jdye64]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Bindings for LogicalPlan::Extension are currently missing and should be added.

Describe the solution you'd like
Add extension.rs to the Python module datafusion.expr so that Python users can use those bindings.

Describe alternatives you've considered
None

Additional context
None