[mlir][sparse] introduce MapRef, unify conversion/codegen for reader (#68360)

This revision introduces a MapRef, which will support a future
generalization beyond permutations (e.g. block sparsity). This revision
also unifies the conversion/codegen paths for the sparse_tensor.new
operation from file (eg. the readers). Note that more unification is
planned as well as general affine dim2lvl and lvl2dim (all marked with
TODOs).