Change ExtensionPlanner::plan_extension() to an async function

[waynexia]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
We have PhysicalPlanner::create_physical_plan() that can build physical plan from logical plan which is an async function. Gives a user-add extended logical plan and physical plan, the planner should also be asnyc like the build-in planner.

Describe the solution you'd like
Change ExtensionPlanner::plan_extension() to an async trait function

Describe alternatives you've considered
Leave it as it is.

Additional context