Consider renaming `UserDefinedSQLPlanner` to `ExprPlanner`

[alamb]

Is your feature request related to a problem or challenge?

@samuelcolvin notes on #11207 #11207 (comment)

Given how much UserDefinedSQLPlanner is being used for existing stuff within datafusion, perhaps it should be called just SQLPlanner or CustomSQLPlanner?

Since we haven't released this API yet, the name could be changed in the next few days before #11077 without causing too much downstream churn.

Describe the solution you'd like

I recommend renaming it to ExprPlanner as it is being used to plan specific exprs which seems good #11207 (comment)

Describe alternatives you've considered

No response

Additional context

No response

[andygrove]

@alamb @samuelcolvin This seems to be the only issue blocking the release. Is anyone planning on working on this? If not, I'll pick it up.

[samuelcolvin]

Please do.