[mlir][Linalg] Add a transform.structured.lower_unpack op

This revision introduces `transform.structured.lower_unpack` which allows
rewriting a `tensor.unpack` to `transpose` (`linalg.generic`) + `tensor.empty` + `tensor.collapse_shape` + `tensor.extract_slice`

The implementation is currently limited to static pack ops that do not have outer_dims permutations.

Differential Revision: https://reviews.llvm.org/D142889