[mlir][sparse] Introduce new reduce op

A new sparse_tensor operation allows for
custom reduction code to be injected during
linalg.generic lowering for sparse tensors.
An identity value is provided to indicate
the starting value of the reduction. A single
block region is required to contain the
custom reduce computation.

Reviewed by: aartbik

Differential Revision: https://reviews.llvm.org/D128004