[mlir][sparse] sparse runtime support library improvements

(1) renamed SparseTensor to SparseTensorCOO, the other one remains SparseTensorStorage to focus on contrast

(2) documents difference between public API exclusively for compiler-generated code and methods that could be used by other runtimes (TBD) that want to interact with MLIR

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D109039