[mlir][sparse] Adding {pointer,index}OverheadTypeEncoding

Work towards: https://github.com/llvm/llvm-project/issues/51652

The new functions fill the gap between `overheadTypeEncoding` and `get{Pointer,Index}OverheadType`.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D122056