[mlir][linalg] Add a FillOpInterface.

Add a FillOpInterface similar to the contraction and convolution op interfaces. The FillOpInterface is a preparation step to replace linalg.fill by its OpDSL version linalg.fill_tensor. The interface implements the `value()`, `output()`, and `result()` methods that by default are not available on linalg.fill_tensor.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D120725