Add a generic loop abstraction to the std dialect

Add a generic loop abstraction to the std dialect

This CL is the first step of a refactoring unification of the control flow abstraction used in different dialects. The `std.for` loop accepts unrestricted indices to encode min, max and step and will be used as a common abstraction on the way to lower level dialects.

PiperOrigin-RevId: 256331795