[mlir][sparse] add asCOO() functionality to sparse tensor object

This prepares general sparse to sparse conversions. The code that
needs to be generated using this new feature is now simply:

(1) coo = sparse_tensor_1->asCOO();          // source format1
(2) sparse_tensor_2 = newSparseTensor(coo);  // destination format2

By using COO as an intermediate, we can do *all* conversions without
having to implement the full O(N^2) conversion matrix. Note that we
can always improve particular conversions individually if a faster
solution is required.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D108681