[mlir][Vector] Introduce 'vector.mask' operation and MaskableOpInterface

This patch introduces the `vector.mask` operation and the MaskableOpInterface
as described in https://discourse.llvm.org/t/rfc-vector-masking-representation-in-mlir/64964.
The `vector.mask` operation is used to predicate the execution of operations
implementing the MaskableOpInterface. This interface will be implemented by maskable
operations and provides information about its masking constraints and semantics.

For now, only vector transfer and reduction ops implement the MaskableOpInterface
for illustration and testing purposes.

Reviewed By: nicolasvasilache, rriddle

Differential Revision: https://reviews.llvm.org/D134939