Add supporting functions for user defined sql planners to FunctionRegistry #11294
Description
Is your feature request related to a problem or challenge?
Currently with the function registry trait you can register a user defined sql planner to the FunctionRegistry but there is no ability to get a list of registered user defined sql planners.
Describe the solution you'd like
fn user_defined_sql_planners(&self) -> Vec<Arc<dyn UserDefinedSQLPlanner>>
Describe alternatives you've considered
No response
Additional context
No response