[mlir][sparse] Factoring out SparseTensorFile class for readSparseTensorShape

The primary goal of this change is to define readSparseTensorShape.  Whereas the SparseTensorFile class is merely introduced as a way to reduce code duplication along the way.

Depends On D126106

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D126233