[mlir][Vector] Introduce the MaskingOpInterface

This MaskingOpInterface provides masking cababilitites to those
operations that implement it. For only is only implemented by the `vector.mask`
operation and it's used to break the dependency between the Vector
dialect (where the `vector.mask` op lives) and operations implementing
the MaskableOpInterface.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D136734