Add support for external configuration #1390
Description
Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As a user I want to be able to extend datafusion-python with third party libraries. This support exists for many things such as table providers, user defined functions, and others. It would be helpful to add custom config extensions that can be used by these external libraries.
Describe the solution you'd like
Add support to SessionConfig to add extension config options. Once registered I should be able to use existing sql commands such as ctx.sql("SET myconfig.my_value=3;")
Describe alternatives you've considered
None
Additional context
This is needed to support follow on work for custom physical plan optimizers.