Add a new dialect interface for the OperationFolder `OpFolderDialectInterface`.

This interface will allow for providing hooks to interrop with operation folding. The first hook, 'shouldMaterializeInto', will allow for controlling which region to insert materialized constants into. The folder will generally materialize constants into the top-level isolated region, this allows for materializing into a lower level ancestor region if it is more profitable/correct.

PiperOrigin-RevId: 266702972