[mlir] [sparse] convenience runtime support to read Matrix Market format

Setting up input data for benchmarks and integration tests can be tedious in
pure MLIR. With more sparse tensor work planned, this convenience library
simplifies reading sparse matrices in the popular Matrix Market Exchange
Format (see https://math.nist.gov/MatrixMarket). Note that this library
is *not* part of core MLIR. It is merely intended as a convenience library
for benchmarking and integration testing.

Reviewed By: penpornk

Differential Revision: https://reviews.llvm.org/D88856