Add virtual methods (isFileBased, filePath, setFilePath) to RimSurface base
class. Implement these in RimFileSurface and RimRegularFileSurface to provide
a common interface for all file-based surfaces.
Replace class-specific methods (setSurfaceFilePath/surfaceFilePath) with the
common interface methods (setFilePath/filePath) in both RimFileSurface and
RimRegularFileSurface. Update RimEnsembleSurface to use the common interface.
This enables polymorphic treatment of file-based surfaces without changing
the inheritance hierarchy or breaking backward compatibility.
Fixes#13296