Rename / simplify `BuiltInWindowExpr` / `BuiltInWindowFunctionExpr`

[alamb]

Is your feature request related to a problem or challenge?

• We have completed converting all window functions to use the user defined API (specifically WindowFunctionImpl). See https://github.com/apache/datafusion/issues/8709

However, there are still a few structures named BuiltInWindow* that now no longer reflect the reality that there are no more "built in" functions:

A BuiltInWindowExpr here:

https://github.com/apache/datafusion/blob/22c1f54411a02009629b3a76a43bd4343add045d/datafusion/physical-expr/src/window/built_in.rs#L38-L45

A BuiltInWindowFunctionExpr:

https://github.com/apache/datafusion/blob/22c1f54411a02009629b3a76a43bd4343add045d/datafusion/physical-expr/src/window/built_in_window_function_expr.rs#L30-L39

Which confusingly is implemented for WindowUDFExpr: https://github.com/apache/datafusion/blob/d840e987cd855fbbdc5d3e5d69683a5b4f279bb6/datafusion/physical-plan/src/windows/mod.rs#L218-L221

Describe the solution you'd like

1. I would like to rename the confusing BuiltIn prefix for these structures
2. Leave backwards compatibility typedefs type BuiltInWindowExpr = ... to ease migration

Describe alternatives you've considered

I would recommend first simply renaming the structs / traits to remove the BuiltIn prefix / module names

Then I also think it is worth seeing if the BuiltInWindowFunctionExpr trait is still needed, or if the code in physical plan could simply call WindowUDF directly

Additional context

No response

[irenjj]

take